I have Q6600 @3.2 GHz and can't complain. But I am not running on the highest settings because my video card(8800 GTS 512) holds me back. If I had better video card I might want better CPU.
My friend runs it on E2220 @3.2 and that does lag a bit and CPU is running @100% constant
any quad core with 2.6 GHZ or higher could max GTA IV easily but u just require at least a video card 9800 GT with 512 ddr3 or higher to equalise the processing data..it woud just be gr8..