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With air, the Q6600 is more likely to clock and run 24/7 since 45nm do not respond well on voltage and running 45nm with insanely high volts 24/7 to keep it stable will most likely kill it.
Clock for clock the Q9xxx series is superior, but the problem is to get them clock for clock. At the moment, and only with a good investment in cooling you can keep a Q9450 near 4Ghz for 24/7 usage. (being lucky enough to get a chip capable, NOT common)
On the other hand, a Q6600 (which is already dead cheap) will clock with a relatively cheap and good HS.
Although I do agree with you I think your logic is wrong.
Yes, clock for clock the Q9xxx series perform faster but it is easier to get a Q9xxx to a higher clock than it is a Q6600.
If you read this forums the new Q9xxx seem to be far more likely to run at say 3.6Ghz than a Q6600 will so in all probabilty you will get a better overclock from the Q9xxx chips with the same cooling.
However, a few extra percent performance in clock for clock speed and maybe an extra 200Mhz on average on stable overclock speeds does still not make it worth buying one since the price difference is £130 unless you must have the absolutely fastest system possible.
It is better to spend the £130 saved on better mobo/memory/cooling (especially watercooling) and get the Q6600. Overall you will for the same money end up with a faster system
Were you read that a Q9450 will clock easy? Remember that running 1.5v through a 45nm chip you play sudden death really.
Everyone is having problems with with the new series chips, mostly stability. Well, if you don't care though about orthos/prime/occt 8 hour long stability then is np.
Q6600. Mine runs stable at 3.6GHz with a Tuniq Tower cooler.