IF I got this HD player... AND rented an HD movie (I assume Block Buster has some on the shelves), it'll still look better than my SD Player and SD Movie, correct?
Nope, they've gotcha there, too.
That RCA upscaling HD player will play ONLY standard DVDs. It will not play either HD DVD or Blu-ray discs. To play Blu-ray discs, you need to purchase a Blu-ray player. (Same for HD DVD. It requires an HD DVD player.)
A Blu-ray player, however, will play BOTH Blu-ray AND standard DVDs.
(FWIW, I think the whole consumer electronics industry has made a terrible mess of the conversion to HD. As you've found, it's damned confusing.)