You can run composite cables from a High Definition Receiver to a standard definition tv, if you want to. The picture is some better that way. Another thing: When Dish switches over to the MPeg4 compression system, everyone's picture is going to improve.
Now, as for tv's: There are a host of different types of HD Tv's out there. There is tube, rear projection, flat panel plasma, and LCD flat panels. The price goes anywhere from about $800 and up. The price depends on a lot of factors. Type of tv, size of screen, etc.
If you stay with Samsung, Toshiba, and Sony, you should do well. There are cheaper brands, but you get what you pay for. If I were you, I would look at cost, size of unit as it will sit in what room, size of picture for viewing distance. Now, if you get a High Def. Tv, do yourself a favor and make sure it has multiple options for hooking up a HD receiver. Look to the future. HDMI, Component, DVI, are all out there right now. And the industry is moving toward full implementation of HD. Be wise. Ask the person you are intending to buy from lots of questions about the inputs and capability of the set you intend to buy. You definitely want a tv that has High Definition Copy Right Protection capability. Or your picture in the future might be down resolutioned to protect the film maker industry from Pirates.
I know there is a lot to consider, and I hope this helps you more than confuses you.
Please feel free to pm me, if you want to.