I have the exact q6600 and Gigabyte GA-P35-DS3L system with OCZ DDR2 800mhz memory.
I am very easily able to oc to 3.0 stable:
1. Turn off EIST (in Advanced BIOS screen)
2. Turn off Auto CPU Fan option (so it is always on not whe the cpu gets hot only)
3. go to M.I.T:
- 9X333
-PCI-Express [set this to 100, never set it to default when OC'ing)
- Performance enhance[standard]
-System memory multiplier [2.50v] - check with your memory to see required voltage, Gigabyte's default volt for RAM is quite low
- set CPU voltage to 1.26 [this pretty much depends on your cpu] - what does the "default voltage" shown at the bottom of the MIT screen ?