Slim. Welcome to the forums.
DDR2-667 RAM's native speed is 333 MHz. That means you should be able to run at 3.0 GHz without exceeding your RAM's limits.
Set your FSB clock to 1333 MHz (333 MHz FSB freq), RAM to Linked, and increase your CPU voltage a little. And you should be able to run at 3.0 GHz.
On my eVGA motherboard, that didn't work. I had to set the memory to Unlinked and manually input memory clock settings. I always run my Core2 boards at 1:1.
You should be able to stretch your RAM to DDR2-733 speeds (367 FSB freq). That will take you to 3.33 GHz. You set the RAM voltage to 2.2 volts. That is a safe voltage for DDR2 chips. Then you relax the memory timing from 4-4-4-12 to 5-5-5-15 or 5's-15 to 6's-18. Leave the command rate at 2T. You will lose a little memory i/o speed (about 3%), but that will have no measureable impact on system performance.
The upper limit of a G0 Q6600 is from about 3.4 - 3.6 Ghz. Anything faster than DDR2-800 is a waste of money. Besides, most DDR2-1066 RAM is really DDR2-800 RAM that has been tested to run at the higher frequency with relaxed timings and higher voltage. And that, you can do yourself.