This has little to do with the Bus. This has to do with the limited ability of the videocard to calculate what instruction it recieves from the CPU.
Then there is no bottleneck if there's no bus operating slower than the information it recieves. A GPU is an independant component, we've gone over this before, it's not a bottleneck, it just happens to be the component that gets put under more load
If you still think a GPU is a bottleneck try running the same game on an X6800, GPUs are much more powerful specialized processors than any CPU which is a centralized logical processor that basically just directs the numerous other smaller specialized processors in your PC.
Like I've said, the only existing bottleneck is the FSB/Mem Bus ratio as the CPU relies or is dependant on extracting instructions from the RAM. If the memory BUS is slower than the front side BUS you've got a bottleneck created as the information starts getting jammed up waiting for the other stuff to get through first. A GPU is never going to have that problem unless you manually modify the clock frequencies of your VRAM and thus create a similar bottleneck between the GPU memory and the GPU itself. If you turn on HDR, AA/AF or try and increase your resolution, it puts more pressure on the GPU. Regardless of video settings the CPU is always performing the same calculations and will thus complete them at the same time regardless of whether it's 640x480 or 1600x1200 or whatever else you wanna cause