According to several technical reviews I looked at a few minutes ago, an overclocked Intel Core i7 system with a single GTX 295 used a grand total of about 330 watts at full load. At idle it was just under 200 watts. This was total power draw at an electrical wall oulet. That means it was total power draw for the entire system with all components. In one technical review the system was easily powered by a high quality Corsair HX620 power supply. Different technical reviews showed slightly different figures but they were fairly close to each other.
A high quality 700 to 750 watt power supply such as those offered by Corsair, PC Power & Cooling, and Seasonic is more than is necessary to power your system.
When I OC'd my Q6600 from 2.4 GHz to 3.6 GHz, current through the CPU power plug went from about 8 amps to 9.5 amps. Considering the power demands of a complete system, the increase that an OC'd CPU needs will be virtually insignificant.