Here is the build for you:
CPU- AMD64 3000- $150
Mobo- MSI Neo-F Skt 939- $88
Ram- 1GB (512x2) Geil Ultra Value- $80
Video Card- XFX 6800GS XXX Edition- $196
Hard Drive- WD 120GB SATA II- $82
PSU- Thermaltake TR2 430W- $43
Case- Powmax CP769PL-6- $43
DVD Burner- NEC Dual Layer OEM- $40
OS- Windows XP Professional Upgrade (academic version)- $90
Total with tax and shipping- $812
This system will be able to run most anything you need. (I would go with a Fortron 450W PSU for $60 if you wanted)