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