Well, I got curious about the actual VISUAL differences between HD-DVD and the upconverted versions of the SD-DVD disks from the Toshiba HD-XA2 player I recently got. The movie Hot Fuzz comes in both versions, fortunately. HD-DVD on one side, and SD-DVD on the flip side, so this made it easy to do a comparison using the same movie.
We all know that moving images on the screen can often look much better than the same image that is paused, so I picked the same spot in two different scenes on both formats and paused the movie there while I snapped a photograph.
Here's what I got as a result.
FIRST screen
SD-DVD version:
HD-DVD version:
SECOND screen
SD-DVD version:
HD-DVD version:
These images are displayed on a Panasonic model TH-50PX50U screen at 1080i. Camera was about 6 foot from the screen using a 105mm lens.
Honestly, from the excellent results I saw from the upconverting, I was rather surprised at the dramatic difference between the images. The upconverting is VERY good, but obviously not nearly as good as the native HD-DVD imagery.