I don't have a wireless network -- the house we live in is wired for Ethernet, and neither of us use a portable computer. My only concern, therefore, is to find a cable that will go from the PowerMac G5 optical audio out to the audio in on iPod Hi-Fi. From what I've been able to learn online, the former is Toslink, the latter -- as you say -- mini. So I'm assuming that the "mini-to-optical digital Toslink audio cable" will do the job.
I confess, however, that most of this information has not been easy to come by. Apple could certainly do a better job here.