I struggle to play GTA IV, none of the textures load properly, and i can never get above 40fps. So, who knows. My friend has the 7750 i think and he gets better FPS than me. :|
The GPU will be a bottleneck. Not just in GTA IV but in any game. - - - Updated - - - It works fine for me, the only problem is that in my case the game wouldn't recognize the amount of VRAM of my 7950 and would set everything to lowest, so I had to use a command at launch to override it. I get 50-60 on highest settings (crap optimization )