DVD players won\'t do hdtv picture justice because hdtv is much more detailed than dvd video can deliver.
One way to show off the picture quality of hdtv is connect to a blu ray or hd-dvd player. Since PlayStation 3 can playback blu ray movies, if you own a ps3 or going to get one later on, you will see the clarity that blu ray movies will deliver on hdtv. when you compare blu ray to dvd picture detail, you will notice just how soft dvd is.
Either playstation 3 or xbox360 games are in high def, hooking them up to an hdtv should show you the picture detail that you wouldn\'t be able to make out on a standard tv.
You can hook up either internal or external antenna to your hdtv via the RFU coaxial connection (the digital one that is), and you watch ATSC over the air broadcast for free. and some channels are in high def.
Or if you have a high definition cable box that have HDMI or component video connection, you can watch digital picture and some are in high def too...