Q6600 is very easy to overclock as soon as you got a good cooling system and a good otherboard...
Seems with standart cooling system, thet it can go up to 3.2GHz in total stability
For Further, big Noctua to cool it or water cooling...
Some have gone a lot further with the Q-series by using liquid nitrogen
But I agree, better take care about the PCI bus speed: I crashed my old pulsar I a few years ago when overclocking my old Duron with a Gigabyte mobo
(And I'm very happy 'cause my old 1999 board is still running, as it was in its box for at least 4 years!!!)