Since you have an SLI board you could put your 8800GTS back in there as a dedicated Phys X card. If your previous card was a 512MB, then a 460 isn't that big of a jump as well. Anyway, that game is demanding on the CPU. Here, you can see how a 5970 scales on it with an i7 at different clock speedshttp://www.techspot.com/review/336-cod-black-ops-performance/page7.html
Needless to say your CPU is a bit below that ^_^. Overclocking it would certainly help, but you may just want to turn down some of the more CPU intensive settings as well.