I agree with Branjo about the Q6600 G0 stepping version. I used to have one. But the overclocking side of it depends on exactly what motherboard it is used in. In particular the motherboards chipset.
I've had two boards in the past. One was a 680 chipset which failed at overclocking my Q6600. I had to replace that board due to other problems which was then a Asus P5Q-e P45 chipset and overclocked my Q6600 with ease and at the cost of not much more Volts. Just need to have a decent CPU cooler really as these Quad cores can get rather hot.
Hope you get it sorted. Good luck