When the CPU is the bottlenecking factor all bottlenecked parts will perform so their speed matches the CPU speed. If you underclock your GPU so it can perform at the same level of the GPU you won't notice any difference in performance because you will be "manually" bottlenecking your GPU instead of letting the automated system do it.
lmfao? you should NEVER underclock a gpu just to keep the system from bottlenexking. gpu are always way ahead of the cpu technology.
even a quad core can not match the 8800gtx is speed and the quad core will drag your system down. hard drives are probably the most hardcore hogs ever.