Once your fps peak the extra cycles will used for pushing windows processes faster like networking etc which will make minor enhancements to the game but not much because some crappy servers can ruin that experience; even if your ping is good.. A good example of that is UT3 - which is one major reason i don't play it too often anymore.
I had my Q9650 at 4Ghz 8x500fsb on this great Gigabyte UD3R board under h20, now i am running it stock and thinking of offloading it to get an editing machine for AVCHD