apple airport express.
they are small devices that plug into wall outlets, they work wirelessly and each can be seen as an "airplay" device.
you can stream audio to one of them, or to multiples of them in different rooms. you can then use any cheap powered speakers you want, or an existing hifi system in that room. I often use PC speakers, or a cheap hifi amp and bookshelf speakers. In my bedroom i have an ipod dock that has an aux input.
I have 3 of them, they integrate seamlessly with itunes and iphones etc