Most HD players have build in scalers. Even the cheapest HD player (HD-E1 at £180) can outperform the best DVD players at this price point like the Oppo 981 or Denon 1930. You would need to spend £500 to £1000 on a SD player to get an equivalent upscaled picture.
It is probably correct to say that for a given price you should get a better quality upscaled picture on a HD player.
There are good and bad exampleof new HD DVD / Blu ray player wrt their quality at playing DVDs. The Toshiba HD DVD players are excellent for the money and give conventional DVD playerss are VERY hard time when playing just DVD, now you can get E1 for £170 even the Oppos justified lofty position looks endangered. If the Toshiba was multi region it would be wiping the floor of the competition in the sub £500 category already. The PS3 is also an excellent DVD player however at the other end of the scale we have the likes of the Samsung Bluray player (1000) which is less than stellar as a DVD player though makes an excellent Blu ray player. It is all down to how the players are engineered.
We have several HD players that are multi region already, and I think some are DVD A as well, can't think of an SACD on off hand though but...Personally I stick with a multi region DVD player and separate HD players, just like I did for DVD A and SACD on quality grounds but I understand the need for fewer boxes for many people.