Every card is different. There is know way to know an exact overclock for a specific 670. To overclock just up the frequency on the core or memory clock, then test with a benchmark like furmark. Once it gets unstable you can either up the voltage or the tone down the oc. Once you get to a landmark ie 1200mhz do the 15 min furmark test instead of the one minute. Finally make sure it doesn't get to hot.