The CPU is responsible for producing the information for the frames. The GPU is responsible for rendering the frames. If you lower the video settings and see an increase in frame rate, then the video card is suffering. If you increase the video settings and the frame rate stays the same, then the CPU is suffering. Every game is different regarding how the CPU and GPU are utilized.
I'm not sure if this is true but I read this on Steam's forums somewhere. If that is true then I'd definitely say it is a bottleneck because I seem to have the issue mentioned above! ^^
I'm suggesting switching to Intel, but the I would have to also buy a new motherboard and buying a new high-end CPU AND motherboard could be quite pricey. I don't have much money so do you guys suggest I get a Intel Mobo + CPU or is there any good AMD (am3) CPUs that could get the job done w/ my 660?