It depends on the speed of the overclock and the task at hand. You probably wouldn't notice much of a difference in simple everyday tasks, but it can add several fps to CPU intensive games and you would notice a difference in other CPU intensive tasks such as video encoding and emulation.
Ah. I see. I have the i7-2600 and I have it set to 3.8GHz. But it seems I am unable to go any higher (I guess since it is not the K version). So this must mean I have a 10% increase in speed? --- Karma... Is a powerful force..