Steve Hoffman, famous mixing engineer, was lucky enough to see a privately-owned 70mm print of the original Star Wars last year. His comments suggest that this version is the best the film has ever looked or sounded. http://www.stevehoffman.tv/forums/archive/index.php/t-141011.html
Here are some 70mm film scans--they'll give an idea of what he might have seen. Some are degraded but most look pretty good. Note the natural colour scheme and great image detail (and lack of horrid blue-cast). http://www.jedi1.net/default.aspx
Interestingly, Hoffman says that the picture quality of the '97 version wasn't nearly as good as what he saw, and that the audio, too, is inferior. Having now heard the 5.1 mix of the '97 ANH, I can confirm that it does not have as much dynamic range or as good of sound quality as the 1993 mix, which is largely derived from the 70mm soundtrack. This has nothing to do with the fidelity limitations of 384 kbps AC3 versus uncompressed PCM; it is due to the quality of the elements used. The '97 version has noise/hissing during dialogue, particularly noticeable in the beginning of the movie, though it clears up later on. I suppose its less dynamic nature is probably due to concerns about the quality of the sound elements if turned up too loud. Given that, I can understand why they wanted to remix the movie for the '04 dvd . . . but they completely botched the opportunity to make a really good-sounding version. I'd far rather listen to the slightly degraded but still mostly authentic-sounding 1997 version than the muffled, distorted, flat-out wrong 2004 mix.
Note that while it is less dynamic than the 70mm version, the SE mix still has higher peaks than the 35mm mix. The LFE channel is used rather conservatively, only coming on strong in a few places, some of which coincide with alterations to the special effects. By contrast the '04 version has very strong LFE in several scenes, though some instances of 1993 bass are not present in either.
The sound elements of Empire and Jedi were in better shape, and the dvd versions are largely unchanged from their '97 incarnations except for specific things here and there.