I'm going to overclock my Q6600 to 3ghz when I get my fancy new case and install my new PSU which should be stable enough to give a decent overclock.
I don't think however that my CPU is being maxed out in many modern games. In fact the q6600 is the best CPU investment I've ever made, as when I first got it very few games supported 2 cores, let alone 4. Now more games make use of the extra cores, and it's really found its feet.