HDTV content? Sounds like someones spending time talking to futureshop sales people lol, no offence. Really the only difference between SDTV and HDTV is the resolution. HDTV is 1920x1080, SDTV is something x 480, nothing else.
So with HDTV you have far more pixels, which means the pixels are smaller, and your image is better looking. Although the pixels don't scale with t.v. size. So a 38" HDTV has the exact same amount of pixels as a 62". From there you have to take into account viewing distance, how far away you're sitting. I don't remember exact numbers, but there's a distance where you can't tell the difference between a 720p t.v. and a 1080p. So if you take that into account, you might not actually need the full 1080p t.v..
The only difference between a DVD and a blu-ray is the amount of data it can hold, nothing more. So with dvd you're limited to 480 lines as you said, because of space restrictions, dvds hold about 700mb. Blu-rays can hold far greater, 20gb+, so you can store more data on it. With the extra room they can fit content that's native to 1920x1080, 1080 scan lines. That content can be displayed clearly at any resolution under that, on any monitor, and any t.v.
It's the upscaling thats a problem, which is why a dvd on a 1080p t.v. will look noticeable worse then a blue ray (depending on viewing distance). There is also no such thing a "HD monitor", pretty much all of the modern ones come at 1920x1080 now, or a smaller size with the same 16:9 aspec ratio.
