I'm just going to give you some fast responses based on a quick read of your lengthy post. The cheapest solution would be to run a second set of speaker wires to another pair of speakers. Not sure why you are thinking efficient speakers unless your amp is really low-powered. You could also interface a speaker switching box as made by Bryston (kind of pricey, but no sonic losses in my experience) and turn off and on each set independent of each other. You'll need an Airport Base Station for your G5 if you choose wireless. The Apple accessory antenna(I believe it may be discontinued) for the g5 towers is absolutely worthless in my experience. Do a search online and you may be able to find some amplified alternatives should you need to use your tower as a remote instead of a base. If you connect the g5 directly to the base station by ethernet cable you don't have to be concerned with buying a card and antenna for it (the base station would be all you need to stream music and to network with your card-carrying iBook). You would need an Airport Express as a receiver if you plan on just buying a DAC with optical input (you'd also need a mini-toslink to go from the AE to your DAC and that DAC must have toslink - Apple makes a stereo-kit for the AE which includes a Monster mini-toslink adapter - there may be alternatives as well). Alternatively you can use the much less than desirable analogue output from the AE to go into another amplifier to accessory speakers, or just use amplified speakers and do away with another box. Not recommended as far as the analog signal from the AE is concerned though. Alternatively you could get a music server device such as a Transporter (which has a DAC) or a Squeezebox or other such device. They'd allow you to interface with a remote control and see your selections AT the location of the actual device, rather than just at your computer.
Hope that helps.
Hope that helps.