When it comes to processors, a general rule of thumb is to buy the fastest that you can afford. Problem is, it's not all just about speed these days. A GHz from one family of CPUs doesn't equal a GHz from another.
There's also the question of how many cores you should get. Entry level these days is dual core, which will suit most people just fine. There are also triple-core and quad-core processors out there, but keep in mind that not all applications take advantage of this extra power. Some video-encoding applications, 3D applications and games will use them, though, so if you're a content-production maniac or a gamer, it'll likely be worth investing in a quad-core machine. If you mainly just browse the internet, then dual core is perfectly fine.
You have a lot of options, so below are the basics.
High performance
Intel Core i7
Now in its third generation, Intel holds the fastest chips in the mobile space. As the performance part, Core i7 is often paired with a discrete graphics processing unit (GPU).
Keep an eye out, though — some are dual core, some are quad core and some have low-voltage chips (meaning lower performance, but also much longer battery life). These days, low-voltage chips are generally found in ultraportable laptops — what Intel calls ultrabooks.
AMD's strategy in the CPU world has changed; it no longer competes at the high end.
Mainstream performance
Core i5
Available in dual- and quad-core configurations, as well as low-voltage variants, the Core i5 is the workhorse of the industry, filling many a fine mainstream laptop. At the moment, both second and third generation parts are in the market (with model numbers starting with a 2 and 3, respectively). While having a newer part is generally better (and in this case, usually brings with it dual USB 3.0 ports), vendors are using the older Core i5 parts to offer cheaper laptops — and, for day-to-day work, these older parts are perfectly fine.
Laptops featuring this processor will either come with Intel HD graphics or a discrete graphics card from AMD or Nvidia, depending on how large the laptop is.
AMD A8
A quad-core part, the A8's performance falls somewhere between Intel's dual-core Core i3 and Core i5, depending on the clock speed.
Budget
Core i3
Now in its third generation, these dual-core processors are almost exclusively paired with Intel HD Graphics, and are for those who only do the basics: word processing, image viewing, music listening and internet browsing. It's a small step above the processors below, and is usually what we consider as being the minimum for hassle-free computing.
Intel Pentium and Celeron; AMD A6, A4h4>
These are low-performance machines that tend to be used in sub-AU$500, 14-inch to 15.6-inch laptops, primarily to keep the cost down. If you're on a strict budget and have modest needs, then these may do the job for you.
Low power, low performance
Intel Atom, AMD C series, AMD E series
These very low performance chips are still around and tend to power 11 to 14-inch machines. While Atom is about to make a comeback in the tablet space, generally, we'd say avoid them — almost any ultrabook will provide better performance.
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