I too had the same dilema. I bought the D1x because it has all the attributes of a pro model Nikon. I wanted a more "solid" feeling camera that would match my f5 and f4s. If you are happy with the n80 attributes than by all means the d100 is a good camera. In image (megapixel) resolution there is "probably" no difference, or a slight upgrade in the d100, HOWEVER, you don't have all the controls that you have with the d1x (ie, iso 125, etc). The same lenses fit both cameras but again do you want the f100/f5 or n80? That's the real question.
Also you can't just go by megapixels or some consumer grade cameras under 1000 are up there also, however they don't have the glass or class of Nikon pro level d1x.
Just my humble amateur opinion. (could it be I'm trying to justify my purchase?)