For socket AM2 there really aren't any upgrades that would really help for GTA IV, he'd need at least an AM2+ board to upgrade to any CPU that would handle GTA IV well. I'd say either consider a new build, or just play GTA IV on a console, the PC version is horribly coded and needs pretty powerful hardware to brute force through the awful optimization.
I would think a Q6600 overclocked to 3 GHz would be able to handle GTA IV. I have an i5 760 at 2.8 GHz with a 6870 and 4 GB DDR3 1333 RAM and the game will hit the occasional lag spike, but otherwise runs relatively smooth at high settings. Even accounting for the i5's better architecture, my CPU shouldn't be that much faster than a Q6600 at 3 GHz.
I for one, am not having any issues with running GTA4 (well, its Episodes from Liberty City) without any lag/ issues! No OC, nothing. There may be that occasional lag, but one or two lag spikes over a couple of hours is something I can live with!