800x600 vs 1280x720 image quality difference

[redsvt]

I have a Dell 1200MP. Not great by all means, but works well with my HD Fury. Its native rez is 800 x 600. In Hi Def at 720p projected at 73 inches (6ft) onto my wardrobe door (space is small) I have a diagonal image of 50 inches. I have the chance to pickup a Mitsubishi HC1500 1280x720 native DLP, for a decent price. Before I commit to the purchase will I see much of a difference in picture quality at this size and distance? BTW from a really good HD source (Pirates/Resident evil/5th element) the picture quality is really good from teh Dell 1200P. There is screen door effect, but its not that bad, and the rainbow effect I have gooten used to and hardly notice, I am wondering if there will be any benefit?