This is one of the most talked about topics on these forums right now, have a search for "media streamers" "Audio streamers" etc.
There are quite a few options, depending on your budget. An Ipod can be connected to a stereo, but a streamer is by far the best option. The best 2 are the Sono's and the Squeezebox 3 (apart from the Transporter, but it's well over £1000), but there are other cheaper options which are good, such as the Roku series. The music is stored on a PC and is controlled by and sent to the player thats plugged into a hifi elsewhere in the house.
Have a look at this thread -
Audio streaming devices
__________________