Check out my setup, it's relatively cheap and is a good gaming machine. HL2 and Doom 3 is crisp and smooth.

DFI LanParty N4 Ultra-D (has onboard sound) - $118
AMD Athlon 64 3000+ - $134
2x OCZ PC3200 dual channel 512mb RAM - $90
XFX 6600GT - $150

I reused some parts like my DVD burner, case, PSU (mine is a 450w by the way). Gonna get a new monitor in a couple weeks though... But yeah so far I'm rocking this setup.

It's air-cooled right now with a Zalman 7700 cpu fan, 2 exhaust fans and 2 intake fans. With that I'm able to overclock my CPU to 2.6. Haven't overclocked my GPU yet though, as I see no need. I may try watercooling soon though so it's not totally out of sight

edit: I also do a lot of 3D work on it and it hasn't failed me yet.