I have the same setup as you propose to do but instead of the kitchen, it my hifi system.
I use the loop module of alsa to stream out from the NAS (the NAS has mpd since it's much more powerfull and I want MPD to be able to stream to the web and do other stuff, on the Pi it would kill it).
You can look here for an almost tutorial -> http://geekboy.fr/cheap-sound-server
The Pi only keeps an open ssh connection with my NAS to stream the music, the bandwith need is so small that it should work on everything !
The only tricky parts in this is to create a good asound.conf for your USB card (I had trouble with mine since it only supports 48kHz )