Which gpu? The overclocking method is different for different generations from nvidia. The newest (gpu boost 2.0 >) already overclock themselves as far as the power and temp limit allows. There's a redundant amount of extra pstates programmed into the vbios, so the offset you set will just offset the frequency of every pstate by the amount you put there.
The advertised "boost clock" is just in typical cooling conditions. It will boost higher at stock with better cooling.
Also, for some generations you need to flash a modded vbios to unlock more voltage.