If you could post your CPU stepping and week we could maybe estimate how far your chip could go. I'm concerned with your only 420W power supply. An OC'd chip can use almost up to 200W or more depending on the speed, that doesn't leave much room for the rest of your components.
For your question, depending on your stepping/week, you may be able to hit 2.4/2.5 with stock volts or close to stock. Chances are you may need a slight voltage increase.