Well, the 8800GT should run MW2 on max settings AFAIK. As for the CPU type, I believe that the AMD Athlon 64 X2 4400+ uses a socket 939 motherboard. That CPU was pretty high end for that motherboard IIRC, and you couldn't upgrade much, just to a AMD Athlon 64 X2 4800+ which isn't worth it. You could add another video card or upgrade it, but then your CPU may bottleneck it. Only thing I can think of is get a bit more RAM.