I have a really old Mac Mini which still has the old PowerPC version of Mac OS X installed. However, as a streaming media server, this is sufficient. You can probably pick one up on eBay for next-to-nothing.
On the Mac Mini is OS X, of course, plus the latest iTunes. That's it! I then have two iPod Touch devices, one for upstairs and another for downstairs. These use the 'Remote' iPhone/Touch application which enables you to control iTunes on the remote Mac Mini, via WiFi. It enables you to choose a track, move to another track, choose speakers, alter volume (if necessary) and more. Dead easy to use.
I then have an Airport Express device connected to any B&O product which I want to stream audio - that could be the Beosound 4, Beocenter 2, direct to the BL5s, the BV7 (with BL9s) and so on.
When I want to play some music, pick up the iPod Touch, choose the Airport Express speaker (you can stream to multiple speakers!) and play. You can then control volume and so on from the Beo4/Beo5.
This whole system is relatively cheap and I don't think it can be bettered (for the price or functionality). The only downside, as you'd expect, is that you can't move to another track using your Beo4/5.