Since it seems like you're looking for what would be the extreme minimum for 3-boxing on a budget..

CPU Athlon 64 6400+ Windsor: $90
Motherboard Asus M2N68-VM mATX: $65
RAM 4GB (2x2GB) Corsair DDR2-800: $57
Videocard eVGA 9800GT $120
Hard drive 80GB Western Digital 7200rpm SATA $34
Optical drive LG SATA DVD Burner $25
PSU SeaSonic 400W $54
Case Generic ATX or mATX: $25
$470

You could cut corners.. but that's as low as I'd be willing to go. Re-use a DVD drive & old case if you need to save $50. Upgrade to HD 4850 for $25 more if you can. I honestly would try to put forth the extra cash for quad core though.


Excludes:
Mouse, Keyboard, Speakers, Monitor, Operating System.
(Cheap headphones for speakers.. generic keyboard & mouse.. total less than $30)