Which cooler are you using? The stock or ACX cooler? For both make sure to keep the temperatures below 85C at 100% load eg. msi kombustor for 15-20 minutes. You should be able to raise it to 1GHz to 1.2GHz if you're lucky. Slowly raise the core clock by 50 MHz increments, and continually run MSI kombustor for 15-20 minutes after you've increased it by 200MHz eg. from 860 to 1060MHz. If it crashes within 15-20 minutes, try firstly raising the memory clock by 100MHz, then raise the voltage. NOTE: It is extremely important to continually check the temperatures, or else your GPU may destroy itself with the power.