Shazam
It seems more or less that the Apple TV is a "Media Center" as it stands... Just a bit small of a hard drive but it also has the apple online service already so you can download for rent at like 4 bucks HD movies with 1080 p output all in one, along with a remote control, and capable of storing itune files and outputing them thru digital out to all the same things as a standalone PC at this point.
So yeah in the end your more or less right a media center would be the best solution, but not much reason to build a pricey pc to do it, again I am only considering such a device, as for audio its one thing to go this route, but now considering you can eliminate buying 25 dollar Blu rays etc... in the future and just easily rent it with a very fast download speed from apple and still have the interface already capable of playing back as a music server this seems the most direct solution to kill 10 birds with one stone possibly.. Also the price seems about right at the bigger units for 300
I think in 24 months once we see the shake out of the blu ray costs, and the major release High def downloads, it will be easy to jam some lossless music files on board as well and still have your PC that you originated all the files from having your backups that would be transferred onto the Apple TV
then you don't need the extra hard drive backup unless you think both your PC and the apple will crash at the same time
I would then not even use the PC for audio playback, just for the original Ripping of CD's, and transfering the files to the apple device etc
Anything else you can download wireless directly to the apple tv anyway.