I would suggest getting a more powerful power supply first. They could probably make due with an entry level card but to be playable something at least midrange (like your 7600) would probably be needed.
Even the 6450 says it needs a 350W PSU. I'm pretty sure any reasonable upgrade will require a new PSU, and all the suggestions above will (mine included)
8600gt or if he has a good dual-core processor or even a quad, he could go with a 8500gt, or even a 8400gs. CoD5 is built practically on the same engine that CoD4 was built apon, so that means it runs almost just like CoD4