There is no static answer, you need to tweak it slowly, and doing so safely will require some patience. You should increase the FSB speed incrementally with voltage. The goal is to get the highest FSB speed (stable under prime95 or similar stress test) with the lowest voltage. Also adjust your FSB:RAM as you overclock, ideal is 1:1.
If you're not so concerned about temperatures, the stock cooler can probably get you up to 2.8-3.0 Ghz. With aftermarket cooling, you can get temperatures low enough to push it more. However going past 1.5 volts is not recommended by Intel. If your motherboard can handle it, a low-end aftermarket cooler will keep it cool enough to reach 3.6 Ghz under that voltage. People have pushed the Q6600 to 4.0+ but going there would require liquid cooling and/or would be putting the chip in danger.