Welcome to the forum.
if you click on the apple (top left corner) then on "About this Mac" and finally click on "more info" you get to see all your machines specs.
Your G5 is running 10.4.11 the final version of OS X Tiger and a good solid operating system.
If you can source an install DVD for OS X Leopard (10.5) your G5 can also run this version, but 10.5.xx is as far as your PPC based iMac can go.
You'll need a retail DVD as an install disk made for another model of Mac will not work.
In most cases it would be to your advantage to max out the onboard memory especially now that memory is so cheap, and your G5 iMac will provide you the best experience it is capable of.