Have you considered the Squeezebox technology from Slim Devices? I've used the Classics before, they stream off your network and each one can be streaming different music. You can jump tracks around too. Logitech bought Slim Devices (last year I think) so be interesting to see how they evolve over the next few years.
There's a free download on there called "SlimServer" that you can set up on a PC to do the streaming. You can also I think get SlimServer running on the Linux installation that comes on Western Digital MyBooks if you hack the firmware.