Not sure about the video.
Are you aware that the audio on graphics cards is only for audio codecs found in movies? You won't get Windows sounds or video game sounds.
I have a lot of video and recommend one of these two devices for some people:
1. Western Digital TV Live HUB, or
2. Boxee Box
The Boxee Box has no internal hard drive, the WD does (930GB usable). I ripped my sisters DVD's to ISO format for her kids and they can play them all from the WD device (140 discs stored).
The Boxee Box has a great interface. You can try the Windows version, but be aware the interface is much improved on the Boxee version.
I'm assuming you could use a hard drive like the 2TB WD Elements USB drive with Boxee Box. I know it turns off and on automatically with the WD HUB. This is an important detail and something you'd want to verify.
This is not true. My HDMI connected HTPC produces all audio for all uses.