I'm going to assume you have a wireless network already since you've mentioned you have an Airport Express attached to the DacMagic. Taking that into account, I think you're really asking (I hope) if you can save the music on to your NAS drive and play it digitally through you're Airport Express/DAC/AMP without using you're computer to play the music - if that is the question then yes, you can do that. You need a computer clearly in order to rip all your music and save it to your NAS drive, but once it's on there you can buy the Apple TV and plug it into your int amp/receiver and also your tv. It comes with its own remote and the Apple TV will pull the music off the NAS drive, so your computer need not be on to use it and listen to your music.
So a computer is needed to begin with obviously as you need a way to get the music ripped and onto your NAS drive, but once that's done, you can use a product like the Apple TV to play your media without any computer involved. There are other products available too, but I'm partial to the excellent interface of iTunes to save all my music and playlists, TV shows and hundreds of movies I have stored on my NAS drive(s) and if you're using iTunes then the Apple TV is the obvious extension to play your music the way you want to. I'm guessing you know this bit but just in case - it will remain digital from the NAS to the Apple TV and then if you use the optical out on the Apple TV and plug it into your DacMagic you can use the DacMagic as your DAC to get better sound quality.