I have a i7 920 at 3.8 ghz, and its very stable. I don't think i can do 4 ghz without one of the coolers listed above. I'm using a cooler master V8, which I got because it was the best value for the money (so i thought). I do regret it a little because I can't do 4 ghz safely, but its almost 1/3 the price of some of the other ones. So if up to you how fast you need your cpu, personally I'm thinking about lowering mine to 3.6 ghz because I don't really even use the extra power.