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.
I've got my Ci7 920 D0 @ 4ghz (With HT), I'm using a TRUE, load temps are 69. 68. 68. 66C
I have lapped my TRUE and its got 2 fans in push>pull, and im using a CM HAF 932 case, so you'll need very good cooling. Mind you theres hardly any differance if performance going from 3.8 or even 3.6 Ghz, I just like a nice round 4ghz.
Currently my i7 920 is at 4.2ghz HT enabled with the Noctua NH-D14 in an nzxt tempest case. From what I've read you can overclock to 3.6-3.8ghz pretty easy with the 920 D0 stepping. Above that and it comes down to your cooling/indivudal chip