The answer is yes it is, and a HD version will look better on HD sets, especially fixed pixel devices such as plasma screen, LCD screens and DLP based technology.
The reason is that the image has to be upscaled anyway to those devices, and external scalers or internal scalers are used to do it. They have to do it in realtime which means they can't used the best methods available as they would be too slow.
If you converted your video up to hidef using the bestest most wonderfullest methods (in technical terms) first, and burnt that to a DVD or file and played that back through your hidef set, the image looks much much better.