This is our resident iPhoto guru, Terence Devlin's, advice:
You can move your iPhoto Library to an external disk or new Mac:
1. Quit iPhoto
2. Copy the iPhoto Library Folder as an entity from your Pictures Folder to the External Disk (that has to be formatted as HFS Extended (Journaled).
3. Hold down the option (or alt) key while launching iPhoto. From the resulting menu select 'Choose Library' and navigate to the new location. From that point on this will be the default location of your library.
4. Test the library and when you're sure all is well, trash the one on your internal HD to free up space.