Most 45nm CPUs should only be set to 1.45-1.5V, anywhere past that you'll be risking overheating, and if your HSF can take it, you can go up to 1.55-1.6V (this isn't recommended for 24/7 usage though).
If you ever go into watercooling, phase, ln2, you can up the voltage up to 1.7-1.8V or higher.