GTA4 will run on a Core2 ok, Just make sure you have at least 1gb of ram on your GPU and have a decent gpu and it will be playable, it will never run great, as it's a terrible console port
i used to run it on a core 2 duo at 3ghz and an 8800gts video card, it ran ok in most parts but some places it slowed down and became quite stuttery,it was borderline playable. it runs better on a quad core cpu.