This game is a unnecessary bitch to good pc's.
A quad core really helps with this game.
I have a e8400 oc to 4.2 ghz
8gb of 1066mhz ram
750 sli ftw mobo
and a gtx 295
and i still dont get over 40fps... (high settings 1200x1024)
Heres a good starter rig, it would play gta4 about as good as everyone else can.