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.
stick with athlon 64 or even a dread p4..
Hey unless you are running a dedicated server two gigs of ram doesn't make sense i have seen a 939 64 3200 with a gig of ram and two 6600gt in sli handle heavy duty BF2 with 64 players at roughly 55 fps (special fps counter was activated.)