I expect it to be as good looking as HD DVD is on my 720p projector--which is to say, excellent (and unless a particular title is not up to par, noticeably better than SD)--when I go "blu" later this year.
PQ at 720p is quite striking! I'm running a Mits HD1000 projector and Blu-ray/HD DVD provides the best picture I've seen to-date. I didn't immediately notice the difference when I first started, however once I went back to 480p there was a very noticeable difference. At first I thought my son had put a bad smudge on the lens. Turns out I was simply noticing the lack of fine detail that I had become accustomed too. Now I struggle to not notice the differences when watching SD DVD.
Actually, an interesting question. On the one hand, your projector already upscales all lower res content to 720P. On the other hand, not all do this with equal success, and no upscaling engine can match the quality of a native 720P [or 1080 I/P] signal. When I auditoined HD DVD an Blu Ray a while back, I noticed a much richer and detailed picture, but my wife didn't see any discernable difference.
In summary, I would expect significant improvement of native material, but only marginal [if any] improvement in upconverted source material. If you're married, don't expect your wife to be impressed.
For SD material you really can't get much more out of it than is already there (480 lines) but I've had it can make really well mastered SD DVDs look very good and consequently poorly mastered titles become even more noticeable.