and hey i have a friend he has the same psu acbel 660 his video card is 4870 and he
says he has no problem with it
It really depends on the temperatures and your setup alot, and another thing is most people wont load both the CPU/GPU to its max at the same time, but I wouldn't take a risk as when a poor quality PSU fails, it might take out some of your hardware also.
Anyways, even with a HD5750, your CPU is goinf to bottleneck GTA4 as it is very demanding. Maybe medium.