The Q8200 is 45nm, meaning lower voltages at stock, and lower voltages for an OC, plus less power consumption. Also, the Q8200 has a 1333 FSB, while the Q6600's in 1066. However, it has a lower clock speed, and a lower Multiplier, meaning a high OC will mean higher FSB than a Q6600. If your board is capable for 450+ FSB, chances are you could get a Q8200 up into the same range as a decent Q6600 (all other variables aside) but if your board struggles with high FSB (400+) then you could probably get the Q6600 higher. Also, the Q6600, unless I am misreading something, has a larger L2 Cache, meaning a decent performance boost.
I, personally, would go with the Q8200, because of its advantages, but the Q6600 is no slouch, either.
Edited by pow3rtr1p - 12/24/08 at 3:50pm