volt mod'n the 7900gt

I got an evga 7900gt currently running at 600/990 (stopped at 990 cuz the slider wont move anymore)

So I got some copper zalman cooler on it with blue zalman heatsinks. I get 50c when its really hot in my room. So if I volt mod this, how much more volate can I get in it and how much more on the core?

--pak