Before you update a caution:
Aperture will need to upgrade your Aperture library and to rebuild it - the library will change considerably. That can go wrong, if your Aperture library has a permission problem or other issues.
Run the Aperture Library First Aid Tools to repair Permissions and Database, before you upgrade Aperture to 3.4.5 (see: Aperture 3 User Manual: Repairing and Rebuilding Your Aperture Library). And be sure that your backup of your Aperture library is current - just in case.
See the release notes: http://support.apple.com/kb/TS2518
Scroll down all the way to "A note on upgrading".