If you ripped CDs via iTunes, Media Center should have no problem importing and playing the files.
If you have purchased music via the iTunes store, it may not be available in Media Center as they used to use DRM to lock the files to their application.
In the last year or two, Apple has since reversed this DRM policy, and re-downloading your music from the store may provide you with a DRM-free copy. (though some albums remain infested with DRM)
Connecting things up via USB seems like the way to go.
If you are purchasing a new machine just for audio, I might recommend something running Windows rather than OSX, as Media Center is more fully-featured on it. (there have been 19 versions on Windows, and two on OSX)
If you want the best quality, you should rip your CDs in a lossless audio format. (FLAC or ALAC)
AAC is a lossy format designed to save space at the cost of audio quality - though for some people it sounds good enough at higher bitrates. (256k)