Keep your Apple iBook G3, buy a 2TB external FireWire/USB drive with RAID. This will be cheaper than a standalone NAS solution, more reliable, more flexible and more powerful and will leverage what you already have. If you want to turn the external drive into a network drive later, you can -- just turn on the Mac's disk sharing on the network and you're done.
It's neither convenient or economical to buy multiple laptops, so I'd stick with one as your music server and either the Duet or the Airport Express. Buy a Monarchy DIP Classic reclocker for each end point to reduce the jitter from either the AE or Duet. Also buy external DACs for everyone and you're set.
Now you just have to choose, Duet or AE?
Fortunately, both systems play well together. You can configure the SlimServer software to point to your iTunes library, and you can use iTunes to play music to an Airport Express. I'd wait until July 11 to see what Apple is releasing for the iPod Touch/iPhone 2.0. Rumour has it there's a new
remote iTunes control software that will rival the Duet Controller.