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*
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?.
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
You can also download the Tiger Installation and Setup Guide
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*
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
Mac OS X 10.5 Leopard Installation and Setup Guide
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