That isn't a bottleneck. A faster CPU will accelerate your GPU depending on the way the drivers are written.
To explain it better, if you were to run a X1800 and get 60FPS and then switch to a X1950 and still get 60FPS - there is a bottleneck.
In your case, the CPU is most likely not bottlenecking the GPU but not helping it as much as a faster CPU.
Maybe in the this case your are getting 80FPS in stead of 60FPS in the same situation but if you have a core 2 duo, you may get 90FPS from the more powerful CPU.