Mac G5 Software update server

Those scripts are meant to interface with a Macintosh running Mac OS X Server, which has Software Update Server as a feature of the OS. If you upgrade your G5 to Mac OS X Server, it should be able to host software updates for the rest of your machines to use via Software Update, which you would configure using that script. A cheaper way of doing this is to just use it as a file dump with copies of all the Combo Updates from Apple Support in a shared folder and just run those locally as a copy of the folder on your G5, however what you describe requires your G5 to be upgraded to Mac OS X Server.

 

Is it already running Mac OS X Server?

 

You don't get a list of Mac Updates, you just have to run Software Update and work out exactly what updates you think youll need (So I have OS X 10.0.4 Combo, 10.1.5 Combo, 10.2.8 Combo, 10.3.9 Combo, 10.4.11 Combo, 10.5.8 Combo and 10.6.7 Combo as well as the updates to iLife '06 that I noted from software update just as I know those are the files I need to update most of my macs to something thats roughly current for that system).

And no, you need Mac OS X Server to use the Software Update Server functionality that is built into Mac OS X Server, and to integrate this into Software Update on regular Mac OS X.