The Q6600 (65nm) was a great chip in its time, a slightly newer model will overclock higher more easily and produce less heat
My Q6600 is stable at 3.6GHz but gets too hot for comfort while doing anything Serious, like folding@home or Prime95 stress testing etc
Newer model Yorkfield 45nm chips maybe can hit 4GHz and still be cooler
It could be said that the QX "extreme" model chips are maybe even now a little overpriced for the performance gains over the regular versions
Good Luck on the upgrades