HardOCP has dedicated an article to overclocking the Core i7 920 processor to discover how much power you'll need when you're running this chip from 2.66GHz to 3.8GHz.
We are going to show you how the power usage of an Intel Core i7 system scales as we move up BCLK MHz as well as vCore and QPI bus voltages from stock settings. These are the two primary voltage knobs you are going to have to tweak in order to get your Core i7 920 stable. And ours has been rock solid at 3.8GHz. While I would like to say, for the sake of e-peen, that 4GHz has been bulletproof for us, it just is not the case. We only have a single 920 sample currently and it is just not stable at 4GHz on air or ambient water cooling, no matter the tweaking we have done on either the ASUS P6T or ASUS Rampage II Extreme.