Mac OS X 10.5 - Technical Specifications
http://support.apple.com/kb/SP517
You probably should add more RAM.
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
The DVD should look like this
Caution - Leopard does not support classic mode. So, if you currently open OS 9 apps in classic mode, you won't be able to do this if you upgrade to Leopard.
Cheers, Tom 😉