Just to back up the earlier post, I have just started using the Logitech Wireless Music System for PC which is now available in the UK - and it works a treat. It's a really simple means of streaming music from Media Center to your Hi-fi. It's relatively cheap. And the sound is really good.
I had previously been trying to use Philips Streamium and it was just too much hassle - constantly freezing, dropping out, unable to buffer the track ahead so there were enormous gaps in playback. The beauty of the Logitech system is it streams exactly what is being outputted from Media Center.
The instructions suggest Media Center isn't one of the media players that's compatible with the system....and that's true if you install Logitech's own software to control it. But Logitech's software is a bit rough and not necessary. Once the Logitech drivers are installed, then it appears as one of the output settings in the Playback Options. Select that...and the output of Media Center goes to your Hifi and the rest of your sounds stay on the PC speakers.
One problem - the Logitech receiver that plugs into the Hi fi has a very basic remote control for track changing (just play, pause, stop, next and previous). It's meant to work with any media player that works with multimedia keyboard controls. I haven't managed to get this to work....has anyone else used the Logitech system and had joy with the remote buttons?