It's better you don't play around with that setting. Your better off to bring in your computer to a dealer, have him check you RAM, and ask if your system supports faster ones.
Playing around with the voltages to add up a few mhz to your system is too risky, you might end up having to buy a new system.
If its' not broken, don't fix it......but if you want to check if your system has optimal components, check it out with someones who knows PC hardware.