Memory, hard drive, and processor are the three most important things for me.
It really depends on what you're going to use the computer for:
- if it's for email and internet surfing, with some Quicken and writing letters on the side, then how fast the processor is doesn't matter. 512mb will work fine, and 40GB hard drive will be sufficient.
- if you want to use it for gaming, or watching videos or tv on your computer, then processor matters, memory matters, and hard drive space matters, and one more thing that matters is video card. You'll want a good memory card that is separated from the processor.
- I couldn't live with a CD burner at the very least, but having a DVD burner is great too.
- If at all possible do a custom build, that way you can get exactly what you want in it and it won't get bogged down with all the unnecessary items that come prefilled or preinstalled. Ask yourself: do you really need that 20-in-one media reader? Do you really need that blinking, pretty pretty lights in the front, do you need the AOL connection, etc etc.
Good luck.