DVD's are definately not as good as HD. Owning an HDTV (50" also)
I have seen this with my own eyes. DVD's do look incredible because they are displayed at their true resolution on an HDTV, not downsized like on regular TV's, so you see what a DVD is really made of. I heard of that up-converting DVD player that brings it signal from 480p to 1080i, and reviews said it makes a hardley noticible difference, nothing as spectacular as HDTV from your cable or satelite provider - not to mention the price of those players .. whew!
But the players may make DVD's look a tiny bit better still don't increase the quality. It may look like it on an HDTV, but you can't create true HD content from a sorce such as DVD. Try and make 640x480 picture to a 1600x1200 one - see where i'm going.
What Kane said sums it all up ... You just can't get quality out of something that isn't there.