Maybe on a medium setting 250 is a little old now maybe an upgrade for Mw3 to see it's epicness. 4GB of RAM is fine could try and put 2GB in there to make it run a little faster. On the whole no major upgrade needed.
personaly i would get a new system as upgrading pretty much any part of the system to match modern demands would essentialy bottleneck the system. if you want a cheap and good gaming system that can run crysis at 40fps check out linus's video = http://www.youtube.com/watch?v=uW1NdcHSUiw