What's the latest OS I can upgrade my Mac G5 with Operating system 10.5.8 to?

I just got this used G5 from a friend so I can start introducing myself to the Apple world. I had enough Microsoft PC issues and I think is time for me to start using Apple from now on. I just want to know what's the latest OS I can upgrade this computer without investing too much money or any at all. My next purchase will be a Mac Pro Quad-core. I will like to use this G5 as the means to learn most of the good things Apple has to offer. Any assistance or advice will be appreciated. I will like to know how much I can do to update and upgrade this old system.

Thanks.

Carlos