K guys,
Not exactly sure what STVS (or was it the other way around?) is but please do not think for a second that standard def tv is anything close to the quality of HD television!!
Think of it in terms of your digital camera where typically you used to get around 3 or 3.5 megapixels per square inch. Now they're in the 7-10 range for good reason. TV is no different in this respect. Standard Definition television is broadcast at 480 x 720 lines where as HD is either 1080p, 720p, or 1080i (i for interlaced which is the higher quality resolution, p for progressive which works better for sports and fast moving images). There is no comparison in video quality here, and if you have an HD set with no HD cable/satellite package then you're really wasting the money you spent on a great tv.
HD DVDs (or Blu Ray depending on which format you chose for your DVD player, one is Sony the other is backed by Warner Bros) will play back on a normal DVD player but it will be "down res'd" meaning it will lower the video quality to play on your standard def DVD/TV Set.
Even if you have an HD TV but a standard def DVD player you will get the lower quality video. Now, standard def DVD is actually a higher quality than what is coming through your normal standard def cable/satellite signal which is typically broadcast at 3.75 mbps, but it is nowhere near the 19-20mbps that you will get with the HD DVD experience.
Sorry I'm rambling here, but honestly even true afficienados of HDTV's can tell the difference between cable and satellite HD feeds as well so when you're talking about SD vs HD its truly a HUGE difference...