GarageBand can be copied over from a successful install to a Mac without a DVD drive. Just copy the main GarageBand application in the Applications folder, then the GarageBand support files that can be found in /Library -> Application Support to the same locations on the other Mac. Since these are huge files, FireWire target disk mode provides for a much faster transfer (or perhaps, IP over FireWire).
Mac OS X Hints
http://hints.macworld.com/article.php?story=20040119122738547