I did the Airport Express and it works very well. It does downscale higher bit rate music, so I now use SPDIF from my iMac to DAC for critical listening, and the AE for my outdoor rig.
That way I have my full library available whenever, wherever I need it. Oh, and the Remote App (free) is pretty cool. Puts iTunes library literally in the palm of your hands. Park the macbook and run it all from your iPhone.