If your 660 is anything like mine then it doesn't like overclocking much. The Memory will overclock great, I can run a + offset of 600mhz all day long, but the clock offset is a waste of time, mine crashes with only + 12mhz. The biggest problem is the locked voltage, at 1.175v maximum its holding things back for sure.
I don't know if you have the 660 or the 660ti, if you have the 660 then it might overclock better than my 660ti, all models vary and brands too, I think the EVGA ones seem to be the worst for overclocking. Msi ones seem to be a lot better.
Edit: Just have a play round with it, you won't kill it due to the locked voltage and if it gets too hot it will just downclock. If you Overclock too high the drivers will just crash and you know you went to far, back down a bit and try again. Use maybe Heaven benchmark to test for stability, usually if it will pass that it should be stable for gaming.