I cant' really speak to the integration and PnP abilities of various NAS devices but I can say that Media Center (MCX) is a great digital media aggregation platform and one I install professionally and use in my house. MCX can be set up to burn music and photo files directly to the local hard drive, or grab them across a network from another computer or NAS. You can perform a simple search across your network from shared folders and files and "watch" them so that whenever anything is added, it's automatically served up in MCX.
You are right, though - there's needs to be some sort of PC-device that the media contect streams through. You can't just serve it up bare bones off of a NAS. Of course, you could use something as simple as Sling or even a MAC mini to get music content off a NAS. You can, however, stream music content from a NAS to an Xbox without the use of a MCX machine and there is an Xbox remote that will give you the CD playback buttons from control. http://www.xbox.com/en-US/media/xbox360media.htm
I also can't speak to degraded performance of HD playback over a network as I've never had an issue. The only issue I've ever had was with a little bit of network congestion where the buffer got full and playback either momentarily stopped or hiccuped in the form of pixelating. This was mostly resolved on my network by changing the gigabit router settings. I'm confident I could 110% resolve it by upgrading to a business-grade gigabit router.
My advice is if you have Vista Ultimate with MCX - use it! You're one and only real limitation is going to be DVD playback - you can't playback stored DVD content via MCX unless you use a hokey, third-party app. I don't recommend that. Hopefully the arcane DRM laws will change soon and streaming DVD content will no longer be an issue for those that don't own Kaleidescape.