One of the great things about digital formats is being able to easily group and find all of your music and video. iTunes combined with Remote.app on iOS devices (iPhone/iPad/iPod Touch) works really well for selecting and playing music around the home or office. Music playback doesn’t need much processing power, so any old PC or laptop can be repurposed for the task, or it can be added to a pre-existing server.
Add a couple of AirPort Express base stations for AirTunes and you can control both music selection and multiple sets of speakers from the iPhone. AirPorts can just be setup to join an existing network, rather than creating their own, to provide an additional set of speakers to send music to.