Upconverting doesn't really do anything.
Chances are, if you have a HDTV tv, it is already doing conversion from whatever source to display at its native resolution, which can be 1080i or 720p, or some 1080p. But the upconversion is not gonna turn some thing lower fidelity to higher fidelity again.
Think of it as this analogy. When you convert a wav file to mp3 and convert it back to wav, the final wav file will be as good of a quality as the mp3. Upconverison does not magically fill in all the missing information in the source materal, it merely pads it so it looks "smoother". When a movie is encoded at mpeg2 on the dvd at 480p, this is as good as you can get, unless you buy the HD dvd version of the movie that studios will release in the future.