I think it also depends on what SD DVD player you were using before. If you had a quality SD DVD player then I think the difference is far less noticeable, even at 100+ inches!!!
If you had a so so DVD player and get an HD player with a quality video processor like the Silicon Optix offering, then I feel that the differences would seem much greater, but not necessarily all attributable to the HD player formats!!!
I just got finished watching Seabisquit on HD DVD and A,B'd it against my own SD DVD version and even with a large screen, I would not want to bet anyone which was which.
As far as audio, the jury is still waiting for a court date. I do not have an AV receiver that supports the new audio standards.