The multiplier is unlocked DOWNWARD.. meaning with the E6600, you could go to 8x or 7x.. but only the X6800 and QX6700 core2 processors have unlocked multipliers up and down.
Edit: Also, you must have a motherboard with a BIOS that allows multiplier adjustment. Meaning to adjust ANY processor downward, the motherboard must have the option, and to adjust the X6800 or QX6700 up, the motherboard must support that as well. Just because you buy a X6800 doesn't mean you can adjust the multiplier.. most enthusiast motherboards have this option though. The cheaper ones (usually $100 or less) most likely don't, although a few do. Read the specs.