Well you can (at least I can, in some games) tell the difference between 60 fps and 120 fps (ie Halo CE), but MW and MW2 aren't it.
Anyways if you got money to burn:
i7 920 WCed and OCed
ASUS P6T Deluxe v2
Intel X25M G2 (OS/Games)
1x 500GB Samsung F3 (Data)
2x ATI 5850s or 5870s or 5970s in CrossFire or nVidia GTX295 in SLI
Corsair 750HX or 1000HX
HAF 932