GTA4 was terrible on my Core 2 Quad Q6600 oc'ed. I would not try running it on a Core 2 Duo. Until i got my 2600k, I never saw above 30fps with a GTX 460 at 1920x1080 with medium-high settings.GTA 4 has issues especially in cut scenes. Need to cap fps to 30fps which the sequences were recorded in. Game play is fine but cut scenes do stutter over 30 fps. GTA 4 doesn't perform well with the default launcher so make a shortcut to the secondary launcher which from memory think it was the gta4 exe. The game's performance does suffer early on despite tweaks but it does improve as you progress.
Regarding cpu, anything from C2D will do.
GTA5 is a far better game but did enjoy 4 and previous chapters. Have played them all.