First, everyones machine has a bottleneck. Everyone has something in their computer that holds everything back. It might be your CPU, or your RAM, or your motherboard, but there is a slowest part in everyones machine.
If you think your CPU is holding your GPU back, test it. At stock, run 3DMark and a game with FRAPS. Record these results, then overclock your CPU. Run the tests again. If 3DMark picked up 200 points, and the frame rate was <5% faster, then there is something else holding the machine back. (could be RAM, harddrive, even the program being benched.) If the jump in scores was a lot, then you have moved the bottleneck from the CPU to something else. Congrats.