Advice for building a media center PC

[NU Star]

Hi

I was hoping you guys could give me some general advice for building a media center PC, which I want to hook up to my plasma. I want to use it as:

-DVD Player
-Video Player for Digital Media
-Audio receiver for my cable box and for playing digital music files
-Streaming video from the internet
-Video Games, mostly ROMs (NES, SNES, N64, PS, etc.), not really any hardcore PC games


Some question I had:

-Is there any reason to get Vista Home, or should I get XP Media center edition, or is there some other better software I should use?
-If I get a external HD for most of the media files, is their going to be any trouble streaming them to the computer?
-What do I need to do to set up the computer as an audio receiver? Can I just get a card with a digital audio input or something?

-Is there anything else I should know/consider?