AnandTech published a holiday buyers guide which helps you choose processors, graphics cards, motherboards and RAM for holiday system upgrades:
If your computer is only a year or two old, wholesale upgrades might be more than you really need. Yes, you can easily find faster processors if you want, but if you already have a dual-core socket 939 Athlon/Opteron you might be able to wait a little longer. If you have a PCI Express compatible motherboard and are mostly concerned with gaming performance, you can probably get by with a new graphics card, some extra memory, and perhaps Windows Vista - what's DX10 hardware good for if you don't have an appropriate OS, right? Similarly, if you've been storing tons of videos and are running out of space, all you really need is a new hard drive. That's an easier and cheaper solution than adding a bunch of hardware you might not need.
We're going to break this Buyers' Guide into a couple parts, in order to keep things manageable. We'll start today with the major system components: processors, graphics cards, motherboards, and memory. The second installment will cover the remaining components and accessories. So let's get to the heart of the computer: the CPU.