What exactly do you mean by 'lag'? That term can be used to describe a few different phenomena.
Are you playing on the TV in your sig? First off, you should set the game to run at the TV's native resolution, which is presumably 1920x1080 (1080p). You should be able to nearly max out COD:BO (although perhaps low or no AA) at that resolution with your graphics card, provided GPU is the limiting factor.
Also, you should install MSI Afterburner and make use of the OnScreen Display capability. Make sure to check the 'show in onscreen display' box for all the pertinent statistics: core/shader/memory clocks, gpu usage, temps, fan speed, memory usage, etc.
Try running the game while you're monitoring those stats in the OSD. Are you clocking up to 3d clocks (675 core I think is a stock 460 in 3d)? Is memory usage < 1000MB? How does the GPU usage look? Temps okay?
BTW, are you playing online multiplayer or single player? If it's online, try the single-player campaign. Does it perform similarly, or much better?