You'll want at least 2 gigs of Ram, probably 4 (though Windows can't use it all, yet).
If you're gaming, you'll want a big hard drive - at least 500 gigs. Depending on your needs, you might want to add a second hard drive, as well.
The processor is where your money will vary the most. You can go with the duo core, the quad core, or the Core 2 Extreme. Stay away from AMD right now. The Intel processors are superior and have been since the duo core technology came out. The price difference between the processors is going to be huge. The Core 2 Extremes run around a thousand dollars, while the duo core and the quad core both can be had for a couple hundred.
Next, your motherboard. Before you get it, you have to decide the brand of graphics card you want. If you want nVidia get an SLI compliant board. That's where you can use two graphics cards together. If you want ATI, you'll want Crossfire.
Onto the Graphics cards. nVidia is, IMO, the way to go, though I know people who swear ATI. If you're going with nVidia, get the 8800's. A pair of those suckers, and you're running DirectX10 and making short work of just about any game out there, including Crysis.
If you're building it yourself, make sure you cool that thing properly. Add fans, freon, whatever. Have fun, cause that will be a kickass system...