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.
My processor isn't the greatest. I'm looking @ an i5 or i7, i want it to be a decent gaming comp mind you. vid card is out of date by a while, looking at like a 4870 or 5870 and ram i wanna get up to 6 or 8gb ddr3. not sure on mobo really. tbh