MDS, I'm surprised you don't want to get a bigger unit (er, wall unit, that is) just to get a bigger HDTV. I've read many times that a good compromise between watching HD and SD on an HDTV is to sit 2 screen diagonals from the set. And I'm a case in point: on my 50-incher, at 100 inches away, HD looks incredible, but SD is still pretty good, on most channels. But of course, that's my HDTV with its SD upscaling. You'd have to look at the SD upscaling of the sets you're considering and factor that in.
Maybe a 65-incher, MDS?
cheers,
supervij