All processors are unique in their voltage tolerance, thermal behavior and overclock potential. A quad core processor will only overclock as high as it's weakest core, however the Voltage ID, or VID, is a rough indication of how much Vcore is required to attain a given clock frequency.
The Q6600 was released with VID's in increments of 0.125 volts, which range from 1.200 (best) to 1.325 (worst). Regardless, even Q6600's with a 1.325 VID are still capable of overclocking to 3.6 Ghz. Overclocking is about understanding Intel's specifications, and not exceeding them.
Vcore Max = 1.5
CPU temp = 71c
Core temp = 76c
You can read your Q6600's VID by downloading Real Temp, then clicking on the "Settings" button.