Trying to upgrade from Mac OS X 10.4.11

I recently was able to revive a Mac that was thought to be dead. All it had was a power supply issue. I would like to update the operating system as much as possible but I can't seem to find an update 10.5 and I can't use any of the higher updates because of the system requirements. The specs of my machine may be found below. please advise how I can download or buy and update for this system. I would like the highest update available but do not know if the system will support the newest update.

Specs as follows:

Machine: iMac G5

Model: Power Mac 8,2

OS: Mac OS X v 10.4.11

Memory: 1.5 GB

Bus Speed: 667 MHz

Any help would be wonderful! Thanks in advance!!