As someone who has done a few of these type of installs in my past, I would reccomend a unit like this:
http://cgi.ebay.com/Denon-POA-3012CI-12-CH-Multi-Zone-Power-Amplifier-NEW_W0QQitemZ350131882551QQcmdZViewItem
Obviously that is an expensive model, and there may be cheaper ones to have, but in my experience that is the way to go. You will find that it is capable of taking input from remote IR sensors so you can use remotes in the different rooms to control volume.