Why my gaming pc not perform better on modern games like gta iv or assasins creed brotherhood?

I think you need a new power supply, this is a very nice system but it is pretty demanding in terms of power especially if you have overclocked components. I recommend a OCZ modxstream 750w psu. But before buying anything turn any clock speeds and timing on the RAM to default and the same with cpu and your graphics card. If the problems persist I would buy a psu. Good luck man :)