Here is a very good article on overclocking the PII X3 720:http://www.microdots.org/ramblings/node/30
But he says to stay at or under 1.425v.
I see a lot of reviews and posts around the web that with good cooling (which I have), it is okay to go above 1.425v, but should stay under 1.500v.
Here is the information for my required voltage.
X3 720 BE (4 Cores Unlocked)
200 Bus Speed and 1600 HT Link with XX.X multiplier
At 1.424v, I have fully stable quad core at 3.3Ghz.
At 1.440v, I have fully stable quad core at 3.4Ghz. (X)
At 1.488v, I have fully stable quad core at 3.5Ghz.
(X) = Would like to use this voltage.