Currently running an Athlon 64 X2 6000+ overclocked to 3.2 Ghz, ASUS EN8800GT (just a regular 8800 GT at factory spec), 2 GB DDR2 667 Kingston ValueRAM (arguably the weakest link in this system, but doesn't make that much of a difference in practice, and DDR2 800 wasn't quite as cheap when I built the thing), Creative Sound Blaster X-Fi Titanium Fatal1ty Pro (most recent addition), along with a Logitech G5 (second revision) mouse and using Sennheiser HD 415 headphones for speakers, all together in an Antec Nine Hundred case.
Some time next year (not entirely sure when, but probably within the first half) I'll be upgrading again once the Intel Core i7s come down in price and there's some new video cards available. My CPU is starting to feel a little bit tired in games these days, to the point where it's actually limiting some, and while AMDs are still solid contenders for regular desktop PCs, for gaming, they just can't compete. My old CRT monitor is starting to die, so I'll need a new one of those at some point, which will likely come with a higher native resolution and thus will require a more powerful video card than the one I've got now.