...if you don't mind putting all the parts together yourself.

Get an NForce 2 chipset motherboard, and an AMD Athlon - the processors are good, and they're cheap. NForce 2 also has some damn spiffy sound built in, so you won't need an additional sound card.

Don't skimp on the fan. Get one of the "upgraded" models - you'll thank yourself later.

Video: This is a weird time to buy a video card. ATI has done a good job of cleaning up their driver act, but there's still some "bugginess" in their drivers - one specific example is that Neverwinter Nights crashes like a Mother with ATI's latest drivers. In addition, they stop updating drivers very early on for "older" hardware - basically, anything they aren't selling as new anymore. You can find hacked ATI drivers that will work with older cards, but don't bet on it. However, ATI has excellent image quality, and better performance than NVidia at this point in the game.