Keep at stock settings, slowly increase core clock by 50 MHZ, then stress test it using Unigine Heaven Benchmark or 3D Mark, if it is stable, then further increase 50MHZ until your card crashes. Do the same for memory clock, too. If you get a stable overclock for gaming, then you can play with the power limit. Increasing it will give more overclocking headroom. But, be careful with the high temperature especially during stress test.