The CPU sends instructions to the GPU. This is a very simple task, considering that the CPU is only telling the GPU what to do, and the GPU is the one actually executing it. If the CPU is already fast enough to provide the instructions, then it all rests on the videocard. Upgrading the CPU to a higher speed will not help you if the videocard is still executing the same amount of instructions. This is why the videocard is the bottleneck in a system, 90% of the time.
Then you have the rare occasions when the CPU is not strong enough. In this case, the CPU is not sending instructions to the GPU fast enough, and the GPU is not executing all that it can. Basically, the GPU is sitting there twiddling its thumbs. If you overclock the CPU, you can increase the amount of instructions sent to the GPU, thus alleviating the bottleneck.
Remember that Game FPS and performance rests mostly on the GPU. This may change in the future with the addition of more CPU-intensive features, and multi-core support. But for now, the GPU rules supreme in determining game performance.