It's been proven on this site and others, that to get the most out of your video card, regardless of what CPU you have, CPU speed has to be 3ghz or above, settling around 3.6 - 3.8ghz range.
You'll want to change all your voltages to their lowest stable settings. Then raise the QDR slowly from 800mhz --> 960mhz, that should get you to 3ghz.