The performance trouble will occurr with the processor. would you try planning fear on a crappy Celeron processor, no.... The semperon is going to hold you back in fps, because the information like AI and other programmable things like that are processed by the CPU, all the graphics information will pass through 6600gt nice and easy but the cpu will start to choke when complex algorithms are need to carried out, the ram will store the algorithms to be carried out to be processed easy.
Celeron and Semperon- don't bother for gaming rigs.