Okay, I think we need to clear a few things up. A Power Mac G5 looks like
this. It contains the
PowerPC G5 processor, but it isn't the only Mac that has used that processor. What you have is the
iMac G5, which
also contains said processor.
Firmware controls stuff like starting the computer, sleeping, and temperature regulation. It is unique to each model and revision of Mac, but most Macs never see more than a couple of firmware updates in their lifetime. These updates almost never add features (only in one case that I know of), just fix problems with the hardware not behaving as it should. Firmware updates don't affect the data stored on the hard drive, so if you erase or replace the drive, they won't need to be re-downloaded and re-installed.
Mac OS X Tiger is software, not firmware. Specifically, it is the operating system that the computer runs. It manages files, runs applications, and so forth. It receives (or received) periodic updates, most of which affect all Macs running Tiger, be it iMac or Power Mac or whatever.
Which version of Mac OS X Tiger shipped on the disks depends when you bought the computer. I believe Tiger shipped with versions 10.4.0, 10.4.2, 10.4.4, and 10.4.7 at various times, just to avoid putting terribly outdated versions on store shelves. In all cases, updating to the latest version of Tiger (10.4.11) is free and automatic. As well, if the computer is more than one revision behind (going from 10.4.9 to 10.4.11 rather than 10.4.10 to 10.4.11), the computer will download what's called a "combo update", which will upgrade any version of the operating system to 10.4.11. That means whether you have the 10.4.0 or 10.4.7 disks, your computer will download the same 180 MB package to upgrade to 10.4.11 after the install completes.
Bottom line? It doesn't matter. Tiger is Tiger, so just run Software Update until there are no more updates to be had.
Where is all of this coming from? I have a horrid feeling you're about to try something rash.