Well, if you get a HD TV, and it is a fullscreen movie and your playing a non-HD DVD it is going to look the same and have black bars on the side of the screen, not unless you get an blu ray or HD dvd player that has upscaling. Then you can stretch the picture without distortion.
To notice a difference you need HDTV, HDDVD/blu ray player, HDMI cable and of course a HD DVD. If not there wont be a difference.
You have seen the PS3 in HD, you know how much more smooth the game looks.
I am getting Ghost Rider on Blu Ray, and when you see it, you will want to touch the screen thinking you can touch Nicholas Cage's sexy six pack.