I have never seen a difference myself, and I'm told I have a very sensitive eye for those type of things. Go to best buy or some place and see if the can show you a 720p tv next to a 1080p tv. I highly doubt you'll be able to see the difference.
Oh, and one more thing, the upconversion DVD players are a gimmick. If you want the real thing, get an HD DVD player for 300 bucks (probably only 2x more than your DVD player). Watching an upconverted DVD is like watching a polished turd