You can upgrade the OS to Tiger 10.4 or Leopard 10.5 depending on which model PB G4 you have. What is your processor speed and how much RAM?
*OS X 10.4 Tiger Requirements*
http://docs.info.apple.com/article.html?artnum=301341
Tiger install DVDs are in short supply. Tiger is no longer available at the Apple Store but may be available for $129 by calling Apple Phone Sales @ 1-800-MY-APPLE (1-800-692-7753). For other sources, do a Google search for MA453Z/A (10.4.6), MA190Z/A (10.4.3) and M9639Z/A (10.4). Also look on eBay. Be sure & buy a retail version (black with silver X) and not a model specific version (gray) (See http://discussions.apple.com/thread.jspa?threadID=1976068 Can I use another Mac's system restore disc on my Mac?.
Also check these web stores: http://www.lacomputercompany.com/cgi-bin/rpcart/index.cgi?command=dispitem&type= sku&sku=19521
http://www.allmac.com/shop/index.php?cPath=5_10003
After you install the base 10.4.x, update to the final Tiger version 10.4.11. Here's the link for the 10.4.11 combo update http://support.apple.com/kb/TA24901?viewlocale=en_US
Look at this link Mac OS X v10.4 Tiger Installing Software Support Page
http://www.apple.com/support/tiger/install/
You can also download the Tiger Installation and Setup Guide
http://manuals.info.apple.com/en/Tiger_Install_Setup_Guide.pdf
When you upgrade the OS, do it via Archive & Install. See About the Archive and Install feature http://support.apple.com/kb/HT1710
Be sure to select Preserve Users & Settings. Also, repair permissions before and after the install.
*Mac OS X 10.5 Leopard installation system requirements*
http://support.apple.com/kb/TA24950
Leopard is no longer available at the Apple Store *but may be available by calling Apple Phone Sales @ 1-800-MY-APPLE (1-800-692-7753)*.
Installing Mac OS X 10.5 Leopard
http://support.apple.com/kb/HT1544
Mac OS X 10.5 Leopard Installation and Setup Guide
http://manuals.info.apple.com/en/leopard_install-setup.pdf
After you install the base 10.5, download & install the 10.5.8 combo update at http://support.apple.com/downloads/Mac_OS_X_10_5_8_Combo_Update
Cheers, Tom 😉