I have gone up to 3.42 on air with a V-Core set at ~ 1.4. I use a GIGABYTE P35 mobo and leave the voltage settings on auto it does a very good job. It leaves me with a temp of around 70C when under a load.
If your thinking about serious overclocking get a zalman, or a watercooler. There is a guide on overclocking the Q6600 at speed-geek.net you might find it helpful.
Good luck with your overclock.