Can I update my Aperture 2 to Aperture 3 or do I have to purchase the new version?
There is no price reduction for the update if you already own Aperture 2, you will have to buy the full version, but Aperture 3 bought from the AppStore is no longer quite so expensive as Aperture 3 used to be. Don't buy the boxed retail version.
Not all Aperture plug-ins are compatible with the newest Aperture 3 versions. Before you upgrade, check, if your favorite plug-ins are compatible with Aperture 3.4.4.
If you decide to upgrade, read the release notes in this document: http://support.apple.com/kb/TS2518
Scroll down all the way to the section
"Additional Information: A note on upgrading your Aperture library".
Don't upgrade without taking some precautions: Repair the Aperture 2 library unsing Aperture 2 before you upgrade and backup the Aperture 2 library and the Aperture application, just in case. The upgrade will require a major rebuilding of your Aperture 2 library.