CPU, RAM, FSB on the motherboard, a fast GPU if a gamer, and an optimized operating system.
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..you have to remember that what you see on the screen, and when you see it is all down to the processes that drive the machine via its internal hardware and the OpSys that's running it.
As far as hardware goes, the fastest CPU with two cores or more is the obvious choice, backed up with maxing out the fastest RAM the 'board will support, and the graphics side of it will always be a lot better/faster with a dedicated add-on GFX card that has its own memory cache - and in general terms, the bigger that is, the better it will be.
Moving to storage drives, the inference is clear; SSD's are a helluva lot "faster" than IDE or SATA discs, but that's not the only criterion - they are still a helluva lot more expensive than conventional disc drives (for what you get) and there is still a question over their reliability over time, plus if they suddenly die, as several of my customers found out, there is no possible way of reclaiming any data that was on them - they are DEAD, end of.
Whether that will change in the future is a moot point; however previous advices to use an SSD for your OS and apps is a good bet, if you can afford an SSD to begin with.
Many people have mentioned Windows' nasty habit of running background services you'll never need - thus tying up resources that could be used elsewhere, and if you're gonna run Windows that's good advice - kill off any processes that don't affect other apps or the systems that rely on them to fire up.
Linux is a far better system than Windows in my book, being far less resource-intensive than the MS offerings, but again it's down to individual tastes as to which OS you prefer on your own PC.
As an old'un who's been playing the PC game for nigh-on 20 years I've found that some of the Linux distroes are by far better than any flavour of Windows, and the ultimate irony of all this is that Linux is free - it doesn't cost you a penny, either for the install OR the upgrades.
Just for reference purposes, this post is being made by by my bedroom PC, a rig consisting of a 9950 AMD quadcore CPU, 16GB of PC1600 DDR3 RAM and a 1TB SATA3 hard drive.
Whichever way you look at it, ANY computer that was designed and built within the last five years is going to be a lot faster than any human brain so unless you're after anything more than that, you'll be waiting a long, long time!
I agree with dirtygirty--I even had to look up au fait (lol). People like me who don't know a lot about computers and ask for advice need more "down to earth" information than the merits of an SSD or Linux. We aren't looking for someone to make all our choices for us, but simply to give us useful information that can help us make a choice. Things like a 7200rpm vs 5400rpm hard drive, decent memory size, type of video card and other things that have a bearing on speed and a pleasant experience. Everyone's idea of "fast" will differ depending on what you do with your computer, so all the little things that can be tweaked to make it just a little faster are probably important to some, but in general the basic things are all that most of us need. Sharing personal experience helps too. Mine was having an older computer (3.2Ghz processor, 512MB of RAM, 120GB hard drive--at least it was a 7200RPM) that over a few years got so full of data it slowed to a crawl. It literally took 4-5 minutes to start or do a restart. Last December I sprung for a new desktop: a Dell XPS 27 All-In-One (Intel core i7; 8GB memory, able to support 16GB; 1TB hard drive; NVIDIA GE Force GT 750M 2GB video card, which is mid-range but quite good). It has Windows 8.1 which contrary to other comments I've read works fine with just a bit of a learning curve. It also is a touch screen which I seldom use but once in a while it's nice to be able to make the print larger with just a sweep of my hand. It was a somewhat "spendy" (over $2,000), but it has all the features to make it extremely fast and visually excellent. It makes me happy every day to have it up and running within one minute and even with all the space taken with software still reacts instantaneously and runs multiple programs simultaneously without a glitch. The picture on this 27" screen is very sharp and I can even watch a Youtube video in full screen mode without it being blurry. I'm not trying to promote a specific brand or feature, but simply giving information on what worked well for me. If a person researches all the basic features that affect the speed of a computer and considers all the programs and features that interest them, and then buys the very best their budget will allow (maybe even going a bit over if there is something they feel would really serve them well in the future), the computer should suit them for a long time.
We would need a more thorough description of what you use your computer for before we could give more specific advice. Since you do say that you like to run multiple programs, then you'll want to spend some of your money on more RAM and a multi-core CPU.
For the vast majority of people, surfing and running Office-ish tasks, as long as you don't get the rock-bottom cheapest computer, then chances are you'll be happy with whatever you get for the next several years. You can buy a heck of a computer for $600 these days.
The only reason you'd want to spend more is if you have a specific task that is likely to grow a lot over the next few years. I'm a gamer, so in order to have a machine that will last a few years I spent quite a lot more. OTOH, my secondary machine, which I use for chat and surfing, is my previous 5-yr-old gaming machine. It's almost exactly equivalent to today's $600 machine. It's far faster than the 8-yr-old Dell XP machine I had been using until earlier this year. I expect it to last quite a while in this role.
You stated that the old XP was your work horse. That means to me you are doing accounting type stuff or maybe engineering. Or any other type of designing. For accounting a multi processor and enough ram memory. For designing add a very fast video with GPU. The Hard drive is not that important unless what you are doing has to save and retrieve data while working.
The only things I'd add to this (for the majority of users, going the normalish non-geek route); Is that if you were intending to deal with a reasonable amount of video editing or photos from a high-end camera (DSLR); then it would be wise to additionally invest in a large, fast hard drive for that data. Usually that means a conventional hardisk and not SSD, as these are still too expensive for most people that need a big disk).
Also, an option I haven't seen elsewhere in this thread, but which can give a significant help to unhampered performance for all kinds of heavier task loads that might stretch your computer (in future if not now); is to have 2 hard drives. One dedicated to the OS and applications, and one for your data. It's much more important when at least one disk is a conventional mechanical Hardisk (as opposed to the newer much faster SSD), because of how disks work. It's enough of a subject to have a thread of it's own, so worth a google search by it's self.
If going the two disk route, and if you have maybe budget for about £80-150 to spend on that speed boost; then put the OS, apps and swap file (if you have to have one), on an SSD. Try to get an SSD over 200GB if going this route, unless you don't mind having to change it in a few years when we get to WIndows 9 or whatever is coming.
Since I'm in my writing flow; I'll add a quick geek advice related to the Software side of things. A great deal of operating efficiency and speed, will come from how well the OS, applications and drivers are designed/written. If you're willing to try something new, or have a friend who can assist in your journey; looking into changing from a Windows world, to a Linux (probably Ubuntu, Kubuntu, or Mint), may give you unprecedented speed advantages, without loss of functionality or even ease of use....but this will depend on the type of tasks you intend to do with the computer, an whether you mind getting your fingers dirty at some point.
You mentioned it would be more work than leisure, so I'm guessing either you will have reliance on some specific brand and niche applications, or you will be mostly browsing and doing basic office tasks like word-processing, and maybe spreadsheets. If you don't have specific brand dependencies or if you don't use specific advanced features in the office apps, then you could easily switch to Linux, save money on licenses and also reap huge speed increases which might give you much more bang for your buck and mean you could buy much cheaper hardware for your needs.
Most of the speed advantages of Linux, are due to the very lean and modular nature of development of the Linux kernel, and most of the libraries utilised in the various applications...this is a big topic in itself, and I'm sure many Windows users will be uncomfortable with the Linux advice, so I'll not oversell it here, and let you research it separately if you feel it might be relevant to your situation
If you run it in Safe Mode, you'll usually see the computer zip along. You can trim out the programs running in the background and add different or upgraded hardware, but eventually the computer will get slower. It is generally a result of the computer shuffling through patch after patch after patch, all while each action is also being reviewed by some AntiVirus or malware scanner. Take all of those patches off and uninstall the scanners, and you'll have a fast machine again. It will just be unprotected against all of the security flaws that were patched, and it will be open to malware.
If you have a real problem with speed, and if nothing is actually broken, you might opt for a bin-internet machine with a printer attached and have internet on something else.
I built my own computer in 2005 and I still use it is a very fast computer.
What I did is I bought a gamers mother board (Giigabyte) that can run 16 gig of ram it has four strips and can handle 4 gig's per strip.
I have AMD 6400 DUAL CORE CHIP and the system cocks all the time.
When I first fired it up running XP I had too much ram and it would shut down. Now running windows 7 64 bit it is just fine running 8 gigs of ram for a system that almost 10 years old not bad.
If was to do it today I would do the very same thing by a gamers mother board and now you can buy a processor and enjoy a fash machine which will last for ever.
I agree with herships that the best you can get is a fast processor, a 64 bit operating system and all the RAM you can load according to what the mother board and BIOS will handle. This will mean a 3gHz or better processor and at least 16 gb of RAM with Windows 8.1 freshly installed on the new machine. Save your files to an external system (I prefer an external hard drive to the cloud). Have an on-board 1 gb SSD for the operating system and resident files and a large allocation for the Windows Swap file. Uninstall all "bloatware" and unused programs. Do not start up any programs you do not need after Windows initially opens, e.g. Adobe Acrobat. Acrobat will load when you need it------a bit slower than if it started with Windows but it will be tolerable unless you do a lot of PDF files. Defrag a platter type hard drive but not an SSD (defragging an SSD cuts its life span).
Oddly; there is a smidgen of truth in here, but in my opinion (and I have 30 years of experience!); If a Windows system is actually running faster in 'safe mode' - you have got MAJOR problems with your setup, and are at the point when you either need to format your hardisk and start installing everything again from scratch, and/or it's time to call an expert in. Safe mode should be running around 15-50% of the potential speed of your system!
If you are replacing a computer with something modern (4th Gen Intel Haswell processor) and a Solid State Drive SSD instead of the old spinning platter hard-drive tech you will be amazed at the boot speed and how fast apps open.
I thought I would miss the huge sized (500GB) hard drive, but I have never came close to filling the 256GB SSD drive.
And free cloud storage keeps my data off my computer - I use Google Drive free 65GB and Microsoft OneDrive 25GB to store my stuff. (I encrypt and password protect my files before before uploading to the cloud).
A dedicated non-intel video card is a nice to have if you play graphic intensive games or edit pictures or video (a dedicated video card can make these tasks faster and look better). I have the intel video card and I don't have any issues editing small videos or playing simple games.
If you find a laptop with 4GB or more Memory (RAM) make sure it has a 64 bit operating system.
I run lots of programs simultaneously (adobe creative suite, microsoft office, project and visio, Dasult systems Draftsight) so I went with 8GB of RAM.
I had a choice between widows 7 and windows 8 and I went with 8 (now 8.1). I never use the "modern" interface so the computer opens directly to the desktop and I replaced the start menu with Stardoc's Star8 app. I actually hate the modern interface and microsoft's weak attempt at Win8 modern but it works fine in Desktop mode. I have disabled the corner gesture and mouse gestures so I almost never have see the modern interface.
I am using the Lenovo Yoga Pro 2 HD (upgraded from the Lenovo Yoga Pro 13 from 2012)
I gave up the original Yoga Pro because the battery life was just about 3 hours.
Now the Yoga Pro 2 battery life is about six hours. (the Pro 2 supports a high resolution (almost 4K) screen)
With a son who grew up in the first truly online gaming generation, 7 years ago for his 18th birthday I paid $7000 for a state of the art liquid cooled gaming machine with 4gb of Ram, a 500Mb graphics card, and twin 500Gb hard disks. These days you could buy a laptop with higher specification for around $600.
Even with his "state if the art" PC, he struggled to hold his own in World of Warcraft battles from Australia with the Americans because of the latency in the Australian broadband.
The point of this now is that for most "punters" even the most reasonably priced PCs these days will meet most requirements. Unless you need to process very high resolution graphics/video/music or want to be a world champion at gaming, then my advice old be to go for a price which is at about 66% of what is available on the open market. Typically 8 - 16Gb Ram, approx 4Gb graphics card and 1 - 2 Tb of disk. This should give ample performance and provide for some "future proofing" as Windows 8 and other programs become more and more demanding.
The other area to spend a little time on is to learn about Msconfig(for Windows) and take an interest in how much "rubbish" starts up when you turn your Computer on using default program settings. By ensuring that the absolute minimum number of applications and processes are activated on start-up will ensure that any PC with these specs will hum along nicely. This doesn't mean that you lose access to the other programs. It just means that you chose when you want iTunes, QuickTime, Adobe, to be started, not all of the time ! On Windows 8 machines I also get rid of (one way or another) all of the "baloney" that is not essential for my basic office and multimedia programs.
I hope that helps. If you want the fastest machines available, then go for a high-end gaming machine and that should only set you back about a third of what I paid 7 years ago.
I would recommend a solid state drive for your Windows operating system. This will make your computer start up rapidly and your system very responsive. Get at least 120 GB in size. Buy a second hard disk drive for your data, as big as you want, at least a terabyte. Make sure it spins fast, like 7200 rpm. If you want a lot of applications open at the same time or to do video editing then you need a lot of RAM, at least 8 Gb, but that stuff is cheap so get as much as your mother board will allow. As far as CPU, this is where the money adds up quickly. Just get whatever is within your budget. Don't listen to people who support a certain brand. Both Intel and AMD make great processors. Usually AMD is cheaper. Don't worry about comparisons. Just spend as much as you can afford. The most expensive processor may be the fastest, but you won't miss out if you buy a processor that has been on the market for a year.
If you were a tech person you wouldn't use Windows as it's the heaviest OS and the heaviest programs are written for it. A light OS with light programs would fly on inexpensive or even old hardware. Anyways just buy the computer you need for the tasks you need to perform.
New. Even an old slow beast is greatly sped up if you clean everything up and re install. But next week it would be slow again, so . . .
CPU. Modern CPUs are fast. really fast. Get it as fast as you can afford, this is and the motherboard is the hardest to redo, so this is where not to save the money.
Motherboard. Same statement as above. 4 Ram slots is great for the future.
Hard drive. Solid state drives are amazing. Just the same if you have to watch your costs this is the logical place to save money, and it is not hard to change.
USB 3 ports. Plenty of speed for us working guys. Gamers will want firewire and solid state hard drives, but for work, USB 3 is plenty.
Ram. Good RAM is matched to the motherboard front side bus. DDR 3. If you need more RAM it is easy to add more. Actual working guys rarely need more than four gigs, but please, leave yourself the option.
Most important thing that is usually overlooked. The power supply. You don't need a lot of watts (it is rare for work to need a fancy video card) but you need a GOOD power supply. Protects your computer better. Leaves open the possibility of later upgrades. Life insurance.