Besides a clearer picture, will an HDMI cable allow full screen for HD Channels?

If you have an HD box and connect it to your tv with an HDMI cable, and access the HD tier of channels, most will fill your screen. Some will have bars on the top and bottom, mostly movies being shown in their native aspect ratio. SD shows and stations will have bars on the sides. And be sure to go into the set up menu on your cable box and be sure the HDMI line is set to 720p when you get your new cable. So to answer your original question, yes get an HDMI cable, but don't spend more than $15.

I have dish and it does

On my TV, I use an HDMI cable from the cable box. (Time Warner... HDTV broadcasts are full screen.) If I go to a non HD program, I tune it in on an analog channel for full screen....For instance, ON TW, channel 4 is NBC in a 4 by 3 analog type display.. For full screen HDTV channel 4 is on 404....So I get full screen on all type of broadcasts.. There are also settings for picture size in the menu, but using a 'zoom' feature is not a good idea for maintaining a good clean picture..

Nope, I'm guessing you have time Warner. The time Warner box does not allow you to "HD" the standard channels, I believe satellite does but if you have cable your SOL, you can change the aspect ratio on the tv but it doesn't work that great.