As above. I don't expect your card to OC any further than 8500mhz, however even going that far in one jump is a bad idea. Stick it at 8200, play some games and see if any glitches appear. Set it to 8300, alt tab back to game and continue, and keep going until you start seeing artifacts and glitches... then wind it back 75mhz and work in 25mhz increments etc etc
When you're about max, pull it back 10mhz for a saftey line, then turn on another few games to make sure it's not that your chosen test game is not using the memory bus 100% and allowing you to clock higher-than-stable... and so you can avoid crashes down the line from any game that does use 100%.