Are your MP3s on your G4? It could be that with your Panther upgrade, your new iTunes is not finding the MP3s. Just drop the files into iTunes and let iTunes re-index those files - that should solve your updating problem.
If you have an iPod full of songs that do not exist on your hard drive, and you want to keep them, you need to get the right applet to allow you to pull iTunes files from your iPod and drop them into your hard drive. I do not remember the name of this applet (there are a few), but they are available and reviewed at www.ipodlounge.com (that place is the epicski of iPod nerds).
Report back on what you did and what worked or didn't, I'm curious as I am not a OS X user, but soon will be.
another Handsome Boy graduate
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