Apple produces quite a cheap way to stream audio into another room/Hifi all you need is an airport express plug it into an amp or a powered set of speakers then iTunes can transmit over wifi to these external speakers.
There is a nice piece of software called airfoil by rogueamoeba which is available for $25 (25 us dollars). It allows you to stream certain application or all audio to these external speakers. You can also select which speakers to transmit to.
There is a new application which I have not seen before, Airfoil Speakers this allows a Mac or Windows PC to be turned in to a receiving Airport express!
There is also an Airfoil Speakers for the iPod Touch! So you get wireless headphones capability