I've got a quad core Q6600 processor at the moment, and it's default clock is 2.4Ghz. At the moment it's running quite happily at 3.0Ghz using air cooling only (Zalman CNPS9500 cooler). Was thinking about pushing it a little more, but my temps are around 55C under load so I don't want to try it long term. There have been some great CPUs this past few years in terms of overclockability (is that a word!?), so I thought it would be interesting to see who else has.