Discovery does rock! You'll notice some "HD" looks better than others. There are a few reasons for the that.
1) Content shot with HD cameras (most of Discovery's stuff and more and more sporting events) will always look the best. Once you've seen Monday Night Football in HD, you can't go back.
2) Existing movies are typically converted from film to HD, the quality depends on how well it's converted, some are btter than others. Example, HD Net shows up-converted Hogan's Heros.. you wouldn't believe how good it looks! The IMAX stuff on InHD is about the best quality conversion I've seen.
3) Delivery. Directv, Dish, Cable. They ALL compress the signal to one degree or another. Some channels get less compressed than others. Most still look VERY good however, but the purists rant BIG TIME about any compression.
The best HD signal is free. Uncompressed and over the air. If you are close enough to pick up your local network channels over an antenna, you can get HD.
This website will help you fiigure-out where your tv towers are, how far and what kind/size of antenna you need. I used a $15.00 "HD" (really just a UHF antenna) in Oklahoma and got a strong signal, based on the info I found at the site. Just enter your address and it'll tell ya what you need. Sweet, free service.
Go here:
http://www.antennaweb.org/aw/welcome.aspx