I'd say a:
Intel E6600
2GB DDR2 800 (CAS4, ie, Corsair XMS)
Motherboard (ASUS P5B or Gigabyte GA-965P-DS3)
7600GS graphics....
E6600: $310
2GB memory: $250
Motherboard: $125-$160
7600GS: $75
Course you'd need a power supply ($40-$50), Case (~$50), Optical drive (~$30), Hard drive (~$80 for a 250GB)