gta 4 is a very bad implemented game it was not made to run at resolutions higher than 720p and it is a very cpu intensive game.
So almost any computer can play gta iv without low fps and lag problems, the mainly problem you are experiencing is because of your cpu, i've compared already, any 2x cores cpu will perform bad on gta 4 with high graphics, so with a 4 core you will be able to increase this performance a little, but i don't think you will be able to max out gta iv with a 6750, just do some tweaks lower view distances, and set some stuff to medium that i think you will be able to play it with high frames. if you wan't to increase graphics more first thing is buying a new cpu but be aware that you will still experience lots of lag.
gta 4 likes quad core cpus and system ram. as it's a poorly coded game and your gaming resolutions needs more cpu power, you could try overclocking your cpu and add more ram.
you could turn gfx settings to the lowest and try increasing them little by little till stuttering starts. use the settings where the game doesn't stutter.
Its the cpu which is holding you. Also GTA4 is gpu hungry game. 1GB vram is not enough for gta4 @ max setting. Try lower setting, shadow quality, especially view distance low, below 50. Keep resource usage to green to get max fps. Good luck.
Agreeing with the previous posters. Your system is not powerful enough to run the game on max settings (or any of the other games you listed).
yes he can play the other games at maximum settings on the resolution of 1400x900...
mw is nothing to gpu, i think i can max out mw3 with my old 9800gtx+, about crysis 2 i could max it out (Without dx11 patches/HD texture packs) with my 560 ti and a e8400 cpu at 1080p so i think that he can max out at 1400x900. just GTA 4 will not be able to run maxed even at low resolution because of the problems already related.