Hmm that's tough. From what I've read, everything past DA PMG4s are digital but that digital signal is through the Apple pro speaker mini jack specifically which is proprietary & is supposed to be used just with apple Pro speakers. I know there are adapter cables for say a Intel Core2duo mini's analog/digital combo plug out to TosLink/SPDIF receiver (buddy of mine uses one of these for his audio/file server in his home recording studio) but I haven't seen one for a PMG4 - doesn't mean they don't exist, I've just never seen one. PM towers are also not quiet with a lot of case fan sound. So if you're bent on using a PMG4, I think some sort of USB/FW DAC would work giving you an interface for TosLink/SPDIF. A MiniG4 would work (small, quiet, cheap) but would also need a DAC as it has no digital audio out.
My buddy Ryan uses a Intel Core2duo mini with good results. They're cheap, they've got a digital out, they're quiet, they're tiny, run em headless once it's set up, and all you need is an adapter cable to send the digital signal to your receiver. If it was me, I think that is the way I would go. It would be cheaper than buying a PPC powermac+DAC on ebay anyhow. If you want an all-in-one solution, maybe a older MBP (one with a mini digital audio out obviously & I'm not hip to the specific models that have that) or something like that.
Good luck