I would never upgrade anything without simulating it first on another platform.
In otherwords,
install a vB3 board on your localhost, test it, and see how you like it. Of course wait until vB3 Gamma or RC are released.
There should be enough evidence floating amongst vBulletin.org and vBulletin.com within the past year to show you why vBulletin 3 is the obvious choice (security, modern code, features, etc). You should decide hands on, though.