With the G4 Digital Audio (and some other models) you can also select the boot volume on startup (provided you have OS X and OS 9 installed on seperate disks or partitions). Hold down the option key when you switch on the Mac and you will very quickly get a "Bootcamp" style screen giving you options for which disk you would like to start from.
Take a look here for more info: http://docs.info.apple.com/article.html?artnum=106178

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That's what I did on my Gigabit G4. I gave a 5 gig partition to OS 9. When I start it up, all I do is hold down "Option" and I get, like he said, a bootcamp-style screen where I can choose between all the OS's installed on my machine.

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