Windows MCE2005 was working fine with this card. I wiped out my windows partition a few months back, so I can't do an A/B comparison at the moment. Prior to Ubuntu, I was running Myth on Fedora. I had quite a bit of tearing during motion but playback was considerably better. Right now I'm seeing choppy playback, slow motion playback at times, and what looks like macroblocking. It's completely unwatchable. My Athlon3500+, 2GB dual channel RAM, 6600GT should be adequate to playback HD.
If I can't get this card working then I'll probably pick up an HDHomeRun and see how it goes.