<< I can say that using the composite cable is the worst way to go. Using that, all the cards will probably look the same. S-Video is the best (for now) and you may see differences between cards at that point. >>
I agree. Dump composite and use a high quality S-Video cable.
I've looked at the exact same choices for my home theater PC. I've tried the TV-out feature on cards from ATI, NVidia, Diamond, and S3. Of those cards, ATI consistently had the best TV-out quality. Unfortunately, of the above listed cards, the only one I've actually used is the Radeon 64MB DDR VIVO, so I can't give you first hand experience with each of them. But since I've done research on each of those cards, I've come to the conclusion that they each have their own strengths and weaknesses.
- ATI has earned a reputation for the best TV-out DVD quality of the popular gaming/multimedia cards. Although I had a few problems getting older games to work with this card, I can say that the DVD playback with this card is really good. IMO it's better than an off the shelf DVD player.
-The GeForce will probably be the most compatable with all types of software. It will also have the best frame rates at lower resolutions. If your using a standard TV with a max resolution of 800 x 600, and your a heavy gamer, this might be a consideration. However, the TV out quality of NVidia cards (including the GeForce 2 cards) seems to be a hit or miss proposition. You might get lucky and get one that's exceptionally good with your television, but if your not happy with the quality of the VE your currently using, I think it's unlikely that you'll be impressed by the DVD playback of a GeForce 2.
-From what I've read, the Kryo II has very good TV out quality but I've never owned one so I can't tell you whether or not it's as good as the Radeon. One area the Kryo II seems to excel at is it's FSAA quality. At lower resolutions this could be useful, but for DVD playback it won't make any difference.
In short, if your main reason for purchasing the card is for DVD playback, you should probably stick with Radeon. If not, the other cards have a lot to offer.