Riva tuner does the job. It's a free application , do a google search for it.
You can check the oc settings they used at gamepc.com and Tom's Hardware.
I do know that gamepc got theirs up to 310/550 MHz (core/memory).
I'm not knowledgeable about all the various settings riva tuner can adjust, but I believe you want to turn Vsync off and anti aliasing on.