I had a very early Power Mac G4 (2000 or 2001) and for older stuff like that, I often refer to
www.apple-history.com which will give you RAM limits, OS limits, and other useful information such as original, unaltered release specs.
From that point, you can go to a used Apple enthusiast site and see if you have the latest upgrades possible if you feel you want to modernize your older machine to its full potential.