Hi all,
I've got my good speakers hooked up to my Linux box now, and I can ssh in and run MPG321 if I want to play music but I'm really after a more elegant solution, i.e. choose track via a web interface. If this isn't possible I'll hack something together with PHP but it'd be hard to believe this hasn't been tried before.
Update, 20minutes later: Yes, it is very much possible. MPD, Music Player Daemon does just what I want.
http://www.musicpd.org/info.shtml for anyone else who's looking to do this.
Keywords in the hope that it isn't so hard to find via google (I went through all these and more):
Remote control music linux, music server remote speakers, remote control network music, network music player.