The performance variation of HDTVs when showing SD content is significant. That's probably why Consumer Reports rates all their rated HDTVs for HD, DVD, and SD separately. It's best to buy one that is tops in all 3 categories. I did that with a new bedroom HDTV last XMas and have been very pleased with watching SD content on that set.
OTOH, my 5 year old DLP, while still an excellent 720p HDTV, is only fair for SD content and I spent money, time, and effort the first couple of years to get it as good as it is. Dish receivers are also a factor, but the ViP211 and 222 seem to be about as good as it gets for SD content from Dish. The old Dish 811, not so much.