I understand your desire to only buy a movie once. I truly feel your pain - I have 300 laserdiscs sitting on shelves in my living room. Some will probably never make it onto DVD, much less any of the blue-laser formats.
To your DVD-on-HDTV question:
I have a 50" Sony SXRD and an OPPO 970. I chose the 970 over the 971 because the 970 has HDMI which makes it easier to pipe through my HDMI-switching receiver. The 970 does very good up-converting to 1080i (the best input available on the Sony SXRD when I purchased it - they may have 1080p inputs by now (?)). I've been very pleased with the picture quality and I don't hesitate buying standard-res (hopefully anamorphic) DVD movies.
Having said that, once you see HDTV at its best, you know that no upcoverting DVD player, however good it is, can provide true HDTV quality - the original bits are just not there. But it's hard not to be thrilled by some of the more recent DVDs - they look wonderful on my HDTV.
I did have a Sony upconverting DVD player (DVP70?) that I replaced with the OPPO. The OPPO picture is somewhat better and provides true blacker-than-black, but I have to say that the Sony ergonomics and player features are currently better than what OPPO provides. However OPPO promises firmware upgrades, so the potential to surpass Sony is there. Plus, the OPPO is DivX Certified!
To summarize: DVD can look very good on HDTV!