I have a Q9450 on an Asus P5E board and get temps ranging from 29 to 32C at idle peaking at 28 under load. Obviously this is at stock. Have only briefly overclocked at approx 3GHz and notice only a slight incr. in temp, approx 38C. Oh yes I am just using an Akasa 965 cpu cooler, in a P182 case.
Pushed things a little harder and got stuck..but pretty sure that was due to the need to increase voltage on chipset. This crash forced my RAID 1 disk set to revalidate...so have held back from further messing aroung.
Overall very happy with this running stock, but then again my previous PC is a P4 1.7 (423 pin) which is 7 years old.
Generally agree that Q6600 sound too good to upgrade, but for new builds, and assuming you are willing to pay the £50 extra to have the chip with the new number on it, the Q9450 makes great sense.