However, before you do, copy plugins, custom brushes, actions, custom help files, etc. from CS2 folders into the appropriate folders for CS3.
Uh, that's ok for presets, brushes, actions but NOT for "plug-ins". The safest way to install plug-ins is to use the installer provides by the developer, not just copying them over. First off, the odds are you'll need updates for CS3 and only 3rd party plug-ins can be moved. Never, ever move Photoshop installed plug-ins between versions. Photoshop installed plug-ins are version specific.