Squeezebox with SqueezeCenter (formerly Slimserver) running on your Mac. It will pick up the music listed in iTunes when you tell it to read from your iTunes DB and it will support Apple Lossless and even AAC as long as it's unprotected.

It's the best for music. The Apple TV would be decent if you're planning to do video. Even then I might still recommend a Squeezebox for the music.