If you can't tell the difference between Component video signal (standard def image) and a HDMI signal (high def image), then you have a setting problem on your TV or an output problem from your provider.
I watch standard def images in "Full Screen" mode on my 42" LCD because I like the screen completely full. Yeah, the video image is stretched a bit to fill the screen but I'm willing to live with that.
When I watch a high def image, the screen is filled and image proportions are normal, not stretched. On some programming, there are black bars to the left and right of the picture to maintain proper aspect ratio of the image, but most of what is broadcast in high def looks perfectly proportioned.
Try this: Connect your cable directly from the wall to the TV, bypassing the cable box. Use your TV tuner to view HD channels. If your local TV stations are broadcasting in HD, you should get a HD picture, full screen, proper proportions and aspect ratio. You might have to go into Auto Programming mode and let the TV program all the available channels. I tried this and it worked great, but I still use my HDTV cable box with DVR.