It really depends upon the stepping revision of your Q6600. Is it G0 or B3?
G0 would have said SLACR on the box, B3, SL9UM.
B3s run hotter, and can be overclocked less.
In either case, not that far. You might just manage 3.0Ghz with a G0, but as Preacher says, keep an eye on the temps, they are a better indicator than anything any of us could tell you.