If you really want to overclock, you should lap the heatsink and the cpu's integrated heatsink. If you do that, you can see temperature drop of as much as 10C. There's plenty of information over at
xtremesystems.
As you can see, the q6600 ihs isn't very flat and has a coat of nickel, which is a poor heat conductor, over the copper. Also, your AC freezer's base isn't well polished either according to my memory.