Can you suggest a device for streaming music from my PC/NAS?

I would probably go with a Airport Express. It is 100 bucks and will stream whatever iTunes is playing over wifi to your stereo. And if you have a iPod Touch/iPhone you can use the Remote app to control iTunes.

You could use a Media Gate. for it but you’re probably getting features that you wouldn’t use if you only want it strictly for music. I’ts probably a little overkill and it is probably more expensive than the Airport Express.

Pulptunes allows you to access your iTunes so long as you have an internet connection. The computer with the music has to be on though.
Mashable did a write-up here.
Works with Mac, Windows, and Linux.