2. Buy a retail Mac OS X 10.5 DVD from a source such as Amazon or eBay. Disks which originally shipped with a computer won't work.
Caution, your compatibility with Mac OS 9 (pre-2001) applications will be dropped if you upgrade to 10.5.
Mind you, you will not gain access to Boot Camp, since all you have is a G5.
If you still need Mac OS 9 applications, make sure to clone your Classic system folder to an external Firewire drive with Mac OS X 10.4.11 or earlier on it.
Backup your data before installing anything new. Dismount and disconnect all media devices before installation.
Your maximum Mac OS X is 10.5.8.