Only the most recent special (Planet of the Dead) was filmed in HD. All the others are SD.I rarely watch anything twice, but I am now watching the Martha shows that were on Sci Fi a week or so ago because the first time I saw them they were in SD what a difference.
Typically Doctor Who DVD's arn't released in the U.S. until after Scifi shows it. So it might be a wait.I have just read in What's on TV magazine a small article that says Planet Of The Dead episode will be available on DVD from June 15th. There was no mention of a U.S.release date but it may well be of a similar time.