Well the Q6600 was a huge step forward for overclocking due to the fact it was the first CPU to use the Intel B3 stepping. A lot of enthusiasts still want theses old kentsfield CPUs because you can up the multiplier to reach higher speeds unlike the yorkfield CPU. Hence why even though they are an older core, they are still in high demand.
Google 'Q6600 overclocking B3' and you can see what I'm ranting on about
