the voltage that you can reach is limited mainly by the temperature of the core
What kind of cooler to do you have?
If its water cooled, than you can hit 1.5v and beyond.
With air cooling its usually best to stay @ or below 1.5v with that processor.
look up your processor @ http://www.amdgeeks.net/overclockdatabase
to see what other people have been able to manage