I went from dual to quad (E2200 - Q6600).
changed nothing else (apart from disabling vsync in GTA4)
I can now run GTA4 maxed at 1680x1050 apart from textures, they're on medium.
I used to not be able to put any sliders above 30 without major slowdown.
The 360 is poo in comparison to pc's, it's better to make small upgrades