It all depends on how much you plan to spend, and how complicated you want things to get.
I would suggest looking at audio streamers first. The Squeezebox 3 is excellent and great value for money. A lot of us around here have them and they are designed to work great in multi room situations. The SB3 does not have an amp or speakers (which is a pro, not a con). For an option which includes a built in amp, look at the Sono's. All your music is stored on your main PC which acts like as a server, piping your music you each streaming device you have. You can have different music in different room's, or they can be synced to play the same music through the house.
iPod's, while great, are not your best option here for a number of reasons - the sound quality will be generally better with something like the SB3 or Sono's, you can have multi 'zones' playing different or the same music in each one, controlling the streamers is much better than the ipod and can in each room (the Sono has an amazing remote and you can use a PDA, laptop etc to control all the SB3's in your home), you will have much for storage on the PC for more music (especially if you use lossless compression (100% CD quality) rather than lossy compressed MP3, with the Sonos/SB3 you will discover a whole new world of joy with internet radio... the list go on really as to why an iPod is not the right choice for this kind of project.
See here -
http://www.slimdevices.com/index.html and here -
http://www.sonos.com/
Also see this thread for an idea of what popular around here -
Audio streaming devices
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