Bit-Tech published an article on Overclocking Intel's Core 2 Quad Q6600. Here's a taster:

What's clear from our experiences is that the Core 2 Quad Q6600 seems to be that new "bang for buck" processor that, with a little bit of overclocking, is going to net you performance that is going to at least match the Core 2 Extreme QX6850, if not surpass it by some margin. I'm sure there will be some that say "you can overclock the QX6850 as well" - of course you can, but that is not the point.

One comparison that we've made in the office over the last week or so since we started really tinkering with our Core 2 Quad Q6600 is to the Athlon XP 2500+ for those of you that remember that CPU. At the time, AMD's flagship processor was the XP 3200+, which was clocked at 11x200MHz. The XP 2500+ shared the same multiplier as the 3200+, but used the slower 166MHz front side bus speed. Getting flagship performance out of the XP 2500+ was as simple as increasing the front side bus from 166MHz to 200MHz - virtually every processor did that on stock volts. I think we're going to see something similar with the Core 2 Quad Q6600 - it shares the same multiplier as the Core 2 Extreme QX6850, but uses a 1066MHz front side bus instead of 1333MHz. Our B3 stepping chip didn't manage to get to 3GHz with default voltages, but it did manage to get there with a slight bump to 1.35V (which is still inside Intel's recommendations) gave us the stability we required. The newer G0 stepping should require less voltage than this to get to 3GHz and there should be the potential for higher overclocks too.