VIDIA's image quality is pretty good for a PC DVD decoder, PureVideo delivered de-interlacing image quality that was equal to and in some cases better than what ATI brought to the table.
Link
AT's article is hardly extensive IMO, I considered it a disappointment at the time.
My personal extensive testing showed that "perhaps" my 6800 had slightly better PQ with DVD playback in MCE using NV DVD decoders. It is very subtle if it exists at all, and I tested it fully using a variety of softwares against a slew of titles.
The 6800 does a good job rendering video, I'm running through some extensive testing with my X800XL right now, and it is looking "very" good.
IMO, the DVD video rendering is a push between them. Unfortunately you have to pay an extra $20 for the nvidia card to get there (the bonus is the nvidia decoders work great and look as good with an ATI card as well) The ATI card has better connectivety and driver support for output to a HDTV, and some of the X800XL cards are VIVO equipped (note that YPbPr output and VIVO are features not available on 6800 cards)
Its almost laughable that someone gives props to Nvidia over ATI for media playback after the Purevideo fiasco, especially the 6800GT which has the broken video processor. ATI's video handling is and has been steller.