Just note that not every Q6600, whether it has a good VID or G0 stepping, will overclock the same. There are some who can't even get past stock or 3GHz.
And I doubt you'll be able to play Crysis at 4GHz, without it crashing. Unless you're using one heck of a liquid cooling system to keep it stable at such a high speed. And even if you can, it's not worth it, to go through all that trouble of pumping alot of voltage to just get it stable at 4GHz. You'll just reduce the life span of the processor quicker, that way. 3.6GHz is a good high overclock, that is easier to achieve stability, and is plenty fast.