Before upgrading to 10.4 or 10.5 with a PowerPC machine, find out if you don't need to upgrade right away. At the very minimum learn how to backup your data before installing anything new.
For any update, it is generally best to install the retail version of the disc that does not say Upgrade, OEM, or DropIn on it.
For some people with non-Firewire PowerPC machines, or no DVD drive, installing 10.4 or higher is increasingly difficult, costly because of the need for a Firewire DVD drive. Macs with Firewire but no DVD drive, generally need a Firewire DVD drive or third party compatible internal DVD drive to install 10.4 or later. Third party internal DVD drives generally need to be http://www.patchburn.de/ compatible, though some by http://www.mcetech.com/ http://www.macsales.com/ and some listed on http://www.xlr8yourmac.com/ may fill that need. Apple did have a limited time available Media Exchange Program CD installer for 10.4, that you may be able to find if someone is selling that, and no longer using 10.4.
For some people Spotlight may be an issue with speed when searching for files simply by file name. I would recommend for people not sure about this, to at least give Spotlight a try after backing up their data, and then downgrad to 10.3.9 or earlier only if it is an issue. Launchbar offers content search within 10.3.9, without interfering with simple Find File:
For some people Dashboard compatibility is necessary, however, all their applications haven't yet been upgraded for 10.4 compatibility, and replacing them with newer applications is not feasible. The third party Amnesty Dashboard Widgets browser helps those found with that dilemma:
If you are stuck with not knowing whether or not 10.4 is needed, you can often partition your hard drive:
after backing it up, or add a Firewire externally or second internal IDE or SATA hard drive to boot 10.4 externally on many Macs to try it out first.
USB drives won't boot PowerPC Macs into Mac OS X.
The system requirements for -
Ask on Discussions if other alternatives exist for you if you are uncertain about what upgrade path you may have.