How to "archive" iTunes files with all metadata?
You can do a manual 'backup' of your existing iTunes library to a separate drive, which then gives you the capacity to restore the library later:

http://support.apple.com/kb/HT1751

It's also possible to leave the library version intact and just move the audio files. iTunes marks missing tracks with an exclamation mark. If you try to play a track that has been moved, iTunes will prompt you to locate the track and enable you to point to it. If you wished, you could eventually manually update all your off-disk music. You can even move your entire media folder, should you wish:

http://support.apple.com/kb/ht1449