Can OSX 10.5.8 build 9L31a be upgraded?

You will need to buy the OS X 10.6.3 Snow leopard disk from the Apple on line store and then software update to 10.6.8. Your machine should be intel core 2 Duo. Be aware, you may need 2Gb of RAM to run Snow Leopard depending on what other things you have running in the background.

Upgrading to 10.7 and beyond is not wise, unless you have all Intel Apps:

http://discussions.apple.com/docs/DOC-6271

A CoreDuo will run up to 10.6.8. Core2Duo with the right applications can run at least through 10.7.5.