A game is a combined effort of CPU+GPU, logically speaking, as someone up here also pointed out, a slower part would let you feel that the game depends on it.
Maybe it just depends on what bottlenecks your system, CPU or the GPU.
It is like for eg. a formula-1 (GPU) and an SUV (CPU) running at top speeds and the fuel (data) is supplied to the formula-1 by the SUV. Imagine what'll happen, formula-1 would burn the fuel run at top speed and wait for the SUV, the SUV would then reach up to F-1 and resupply the fuel. If the SUV is a Hummer and they are fast then SUV replaced by a Ferrari would make them even faster.
I hope it's a good example Edited by imran27 - 9/3/13 at 5:35am