Depends on chip to chip...you have to do a trial and error sort of thing.
Keep the chip @4.7GHz, disable speed-step.
Lower the voltage by a very small amount....and see if the system boots. If it does, test for stability using Prime95.
Repeat the above steps, till you start getting errors.
Increase the voltage by one small level, and that shud be your least stable voltage. Test one more time to be absolutely sure