Read the June 2008 issue of Maximum PC's Deathmatch: Q6600 vs. Q9300.
Using a 790i ultra the Q9300 was the better overclocker able to hit 3.5 on air "without a sweat" according to the article ( read it and weep kids dont argue with me) and the better overall performer.
Link plz, I can't find it. Oh, and theres more to overclocking than temperature of the cpu. No doubt at any speed the q9300 would run cooler, but the chipsets starts becoming an issue at that fsb. And the voltage? Putting 1.4 volts through a q6600 is safe, yet that voltage through a q9300 would quickly degrade the processor due to its 45nm architecture. In short, yes, with a good motherboard it is easier to achieve higher speeds with the q9300, but for the average user using it in an everyday comp for 2-3 years, the q6600 is the better choice.