I've had/got a fistfull of G0 Q6600, and in the best of mobos of the era, less than half of them went over 3g stable.
I have to put together an IP35-Pro and a Q6600, if it goes to 3.1g I'd be extremely pleased. 3.4 would be my max expectation.
The G41m is a fantastic overclocker, for what it is. £30-40 you can get it for. If it takes a Q6600 to 3g and not over, I'd be very impressed. Being able to massage to voltages accurately is the biggest downfall.
I've got a Q94xx in one doing around 3.4/5 to which I have to set the voltage at 1.4+ in the bios, although you get to the desktop reading tools and it'll only tell you what the cpu is being allowed to use from what it's asking, which is alot less and not level.
At the same time, you can lose power to a pci slot by overstepping the mark.
They're not the easiest to work with, but they are good overclockers. Overclocking isn't always easy like on the newer mobos.