I would not look at the q9300. At that point they would be very close, although I think you would find the q9300 a little better because of the slightly more advanced core microarchitecture. But, with a multiplier of 7.5, you will be limited from overclocking. You will not hit the processors potential because you may find you are fsb limited. The q6600 will he heat limited.
If you look up the q6600 and q9450, you will see the same argument. In you case, it will be amplified because you are running a multiplier of 7.5 vs. 8 for the q9450.
In total honesty, look at the q9450. It has more cache (this is where Intel finds they get a good speed boost from) and is just faster (by ~.1GHz) then the q6600. The q9450 is thought to be limited by the fsb while the q6600 is limited by heat (you are hitting the processors potential... maxing it out).
This what I have understood from a few other threads. Like aznguy0028 said, GO WITH THE Q9450!!! You will not regret it!
If I have said anything wrong, please correct me.