Well I have a i7 860 and at the moment I am running at 3.5GHz (166x21)
I ran prime 95 for a few hours and it was stable and the temps on the cores stayed below 70*C at 100%. So I'm pretty happy with it being sufficient OCing and being stable.
I did however get it up to 3.9GHz with 186x21 and ran that pretty much just so I could try and beat my mates score in 3dmark after hitting the wall with my HD4890 as far as overclockability.
I tried going for 4GHz, but it would get to the windows XP loading screen and just freeze, and with my temps already up in the top range from the previous tests I decided it was time to call it a day and be happy with what I have.
If you are going to be overclocking, you really need to get a cooler for it. The stock intel cooler is useless.