Personally I would just make my own GUI and use a 3rd party device to stream the media off a mini server. I Don't like the fact the home owners computer has to be on and running a media server with Autonomic. (But thats just me)

Edit:Hmm just noticed you were asking about a HTPC, In that case I would try soemthing like urc PSX-2, even tho I havent had experience with it yet it "says" it can stream your iTunes music with meta data for remotes and GUI for on screen navigation, what I'm not sure about is if it can handle streaming your stored videos