A Q6600 G0 CPU should be able to run at 3.0 GHz with little or no increase in vcore. I bet you are inadvertently overclocking your RAM and that is limiting your CPU overclock.
First, raise your RAM voltage to 2.2 volts and relax the timings to 5-5-5-15.
Then go into the BIOS and change the System Memory Multiplier (or whatever your BIOS calls it) from AUTO to 2.00, 2.00B, or 2.00D - whichever you need to set the Memory Frequency to twice the FSB. Then when you increase the FSB, the memory clock will rise in step with it. If you are at 3.0 GHz (333 MHz X 9), your memory clock should be at 667 MHz.
Download CPU-Z to check your FSB:RAM ratio.
Warning - confusion factor between what the BIOS calls things and what CPUZ calls things. What the BIOS calls "memory frequency" is actually the memory clock. What CPUZ calls "memory frequency" is half the memory clock - DDR2 RAM, remember? It transfers two chuncks of data each bus cycle. What you want in CPUZ is a 1:1 FSB:RAM ratio.
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Overclocking since 1978 - Z80 (TRS-80) from 1.77 MHz to 2.01 MHz