Hmm... I'm not sure about that. I would have thought an X800 would have been enough as it does contain some hardware acceleration support for HD video playback.

I'm sure I used to run HD content from youtube on my nVidia 7800GT. When I get time I will boot up my emulation system that is now using that card and run a HD youtube video and let you know how it manages.

Are you sure it wasn't your connection being slow and causing the video to slowdown? Even fast connects can suffer at peek times.