overclocking intel d940 dual core 3.2 ghz cpu

hello, can anyone tell me how to overclock my processor it's currently running at 3.6 ghz and i have been told that it can be overclocked even more even upto 4ghz. I have a asus pw5d2 premium motherboard which allows me to overclock my cpu using AI booster. I have tried different overclock settings but they seem to fail overclocking test. Do i need to alter cpu voltage?. CPU temps are at around 50c and stable when using my silent water cooling kit.
Any help or suggestions would be most welcome.#

goodie69