I then used that screenshot to make 2 high res JPGs. One with 800 lines (So slightly stretched 720P) and one with 480 lines, what you would get with broadcast TV.
Viewing them both fullscreen (The full res one at 100% and the 480 lines on scaled to fit the screen) there is VERY LITTLE percievable difference between the two.
Sure you may see a noticble differnce with a very large screen, but from what I have seen, there doesnt seem to be that great a benefit in terms of percievable quality of HD over SD.
Tests hardly scientific, and source material (.mkv BluRay rip) probably don't help, but this is how it seems to me.
(Screenshots of my screen showing each of the images: Full Res | 480 Lines)