Q: How do I switch the start up disk on my G3

Hold Option on startup to select which volume to boot into.

Try holding X on startup to force boot into OS X with the proper volume.

Reset PRAM.

If you still can't boot, then you need to reinstall.

http://support.apple.com/kb/TS1892

Some need a firmware update to get past 10.2/10.3...

Apple firmware updates...

http://docs.info.apple.com/article.html?artnum=86117

G4 only...

http://docs.info.apple.com/article.html?artnum=120068

Tiger Requirements...

To use Mac OS X 10.4 Tiger, your Macintosh needs:

    * A PowerPC G3, G4, or G5 processor

    * Built-in FireWire

    * At least 256 MB of RAM (I recommend 1GB minimum)

    * DVD drive (DVD-ROM), Combo (CD-RW/DVD-ROM) or SuperDrive (DVD-R) for installation

    * At least 3 GB of free disk space; 4 GB if you install the XCode 2 Developer Tools  (I recommend 20GB minimum)

http://support.apple.com/kb/HT1514

See Tom's, (Texas Mac Man), great info on where/how to find/get Tiger...

https://discussions.apple.com/message/15305521#15305521

Or Ali Brown's great info on where/how to find/get Tiger...

http://discussions.apple.com/thread.jspa?messageID=10381710#10381710

Leopard requirements/10.5.x...

    *  Mac computer with an Intel, PowerPC G5, or PowerPC G4 (867MHz or faster) processor

minimum system requirements

    * 512MB of memory (I say 1.5GB for PPC at least, 2-3GB minimum for IntelMacs)

    * DVD drive for installation

    * 9GB of available disk space (I say 30GB at least)

Trouble is Apple no longer sells it, check eBay & such for the Retail version, not the Gray Discs...

http://www.ebay.com/sch/i.html?_nkw=mac+os+x+leopard+retail+10.5

There are workarounds if the 867MHz is the only hangup...

http://sourceforge.net/projects/leopardassist/

http://lowendmac.com/osx/leopard/unsupported.html

List of Applications Not Compatible with Leopard...

http://guides.macrumors.com/List:Applications_Not_Compatible_with_Leopard