I'm comfortable with clocking my Q6600 as long as it doesn't reach 70C under full load, which it didn't do when I ran it at 3,375 (9x375). However, when I tried 3,4 with a multi of 8, it wouldn't run stable at all, so I figured it's best to just raise the FSB without changing the multiplier. I had it running through a 3Dmark06-test at 3,6 with no problem at all, but that was just a fun-run for benching, and I didn't set the vCore manually.
Is it the B3 or G0 revision of the Q6600? Remember, it also comes down to the mobo and BIOS, so don't expect to reach the same results if your mobo can't handle it. Also, I would think the stock cooler could handle it, as long as you don't mind the noise.