Right now i am speaking from experience, i just purchased the Q6600 and it is overclocked to 3.2GHz. I am incredably pleased with its performance. The 3.2GHz on my motherboard is a light overclock, with the G0 stepping this beast is begging for 3.6GHz or more. the E6850 will cut out at around 3.6GHz. If heat is an issue get a very well ventalated case and a good HSF. In my opinion go passive, and strap a large/powerful fan to it. Since you will be overclocking the Q6600 can overclock quite a bit higher than the E6850, and for the same price you will get 2 extra cores. But if you are truely interested in a dual core, go with the E6750.