I agree with people's comments on the Thermalright 120 Ultra Extreme, a very nice sink all told. My latest PC was built with one of these and to start wit I was actually running it passive (no fans) as it's actually based on an older design intended to be run that way, it kept the CPU cooler than a stock cooler would have done.

When I decided it was time to push the boat out, take the bull by the horns, and mix metaphors with some heavy-duty overclocking I strapped a couple of Flex 120mm fans to it (meant to only have one but I kind of cheated..) and it happily kept things fairly cool as I was pushing the Q6600 to it's limits.

Can't remember what speed I eventually ended up with but think it was in the order of 3.3GHz which isn't too shabby for a 2.4GHz part.