You should be able to get it to ~ 3.2 without too much effort -- I would use that as your first stable target. I found that anything more required me to throw lots of voltage at it. My propus seemed to settle in at 3.47 and 1.4v.
There is lots of info in this forum on the propus chip. If you spend 10 minutes searching on this site you will probably find a much more detailed answer.