Efficient speakers only because I have 4ohm speakers that demand lots of juice…if I upgrade the speakers in my main room and move the 4ohm speakers to a Satellite room, long wire runs (even with heavy gage wire) could demand more juice than the amp can output.
Understood.
So, the Apple Extreme is NOT recommended for this application?
The Airport Extreme is not recommended in using the line output option. That sounds terrible to my ears...unless you have low expectations of the satellite system and are only listening while doing other things I would not go that route personally. If you use the digital output option to an external DAC you'll be getting better results, IMO, depending on the DAC you use. Again, this is only an option via optical/mini-toslink and NOT via other connective routes.
Simple solution for the getting the first pair of remote speakers up and running:
I just buy an Airport Express base Station and an Ethernet cable.
Connect AE to the G5 with the Ethernet cable (no other hardware/needed?).
Run speaker wire to a Satellite Room #1 with a pair of monitors.
If you run speaker wire from your amp to another room you do not need anything else but a pair of speakers to hook them to. If you are referring to powered speakers (speakers with amplifiers integrated into them) then you would still need a receiving unit in the remote room. This unit could be an Airport Express with line outputs as I suggested sounds just OK, but would not meet with high-end expectations, IMO. It could also be a unit like a Squeezebox, which actually displays the songs at the unit and has a remote control, or better yet, a Transporter, which actually integrates a decent DAC. Otherwise you would need an external DAC to take the optical feed from your Airport Express.
At that point, will I be able to remotely control iTunes playing on the G5 from my iBook (with AE card) in the Satellite Room?
You could use a remote with a Squeezebox or a Transporter, but with the Airport Express options you would need to be controlling your selections from your base computer, or from your laptop if it is properly networked via the Airport card you indicated it has. I'm not sure about that networking interface since I do not use it myself. Perhaps someone else could advise here. With the Squeezebox or Transporter they have their own remotes and you see the selections on the respective units. They hook up wirelessly to your computer/music library (on your external hard drive you indicated you had).