yes, I understand the point you are making about bottlenecks - the question is to what degree the CPU (or GPU) is a bottleneck.
I suspected that for my old system the 1.7ghz P4 processor was a bottleneck for the 6600GT that was coupled with it.
Depends on the application and screen resolution.
If your GT was running COD4 at 1920x1200 then it would be the limiting factor.