Do tests with FurMark for checking for errors after overclocking. Go up in small increments, check stability as you go with 5-minute tests. Once you get a crash, see artifacts, or get FurMark errors, back off to the last stable test point. From there, drop back by 30-50mhz, and run FurMark overnight to test stability. If it's not stable, back off some more. Remember that you want to be sure it's stable. Also, watch temps -- GPUs are designed to run hot as hell, but you don't want it to breach 90 (100C is the usual design window, and you should always leave some wiggle room for hot days and degradation). Crap fans will give you limited room for overclocking, obviously.