If you are just overclocking the CPU (E8500, 65 watt TDP), you will need less than 20 watts more from your PSU.
When I was overclocking my Q6600 system, I was curious about what that would do to system power consumption.
Q6600 VID = 1.2625 volts. Core voltage at 3.6 GHz = 1.425 volts. Measured 12 volt current, stock BIOS settings = 8.5 amps (96 watts) into the motherboard power regulator. OC'd = 10 amps (120 watts) running at 100% load. CPU power consumption increased 25%.