Honestly nothing you can do, other than a massive change in drivers(like from 260.89 to a 270.61 in the 4xx series), will improve your GPU performance like overclocking, especially in cases where you are using more than one GPU. However new drivers may go largely underutilized if the CPU can't process information quickly enough. You can try to play at a higher resolution or generally lower graphical effects as well.
My project on this entire subject will start shortly and should be completed in the next week:http://www.overclock.net/pc-games/1002182-analysis-cpu-frequency-its-impact-gpu.html
but you can see the increase in performance just from a slight overclock here:
This is a sample test from my upcoming project. With a 17% overclock average frames per second went up 22%!Edited by Lampen - 4/29/11 at 1:07pm