Just so you know I went from my overclock Q9400 (400x8) on my P5Q Pro board to an i7 920 and P6T6 X58 board and GTA IV was night and day. What the overclocked C2Q struggled to do at 30fps the i7 920 at stock speeds blazed through at ~50fps. 25-30fps was minimum frame rate.
I'm not kidding when I say it was night and day difference. I used Fraps before and after so no guessing on my part. All other components around both stayed the same (4870x2, 300GB Velociraptor, etc except for the obvious ddr3 needed for the i7).