same goes for vid cards as does for cpu's. The memory clock is pretty easy to start with. gradually bump it up in 10-20mhz increments and run a benchmark like 3d mark.(I am against running furmark as it places unnatural stress on the vidcard and can cause them to fail.) you should get to a stage where things either crash, or you get ramndom screen artifacts. once you reach this point back off the clocks till its stable. once you have one of the clocks stable, now change the other. You should forget about voltage increase for the time being. Read some 6850 reviews, reviewers will tell you the approximate max frequencies they were able to achieve. this will give you an approximate limit, although every chip is different.