The D1x has an unusual CCD with a resolution of 4028x1324 pixels - i.e. twice the horizontal resolution you'd expect given the aspect ratio. When it generates JPEG files this is interpolated to give a 3008x1960 pixel image.
The D100 has a conventional CCD and produces JPEG files with a resolution of 3008x2000 pixels.
When reviewers shoot test charts there is a noticable difference in the resolving power between the two cameras: the D1x does better horizontally and worse vertically (as you'd expect given the design of its CCD) than the D100. In practice, with real-world shots, you might just be able to see the difference if you compared shots from both cameras side-by-side.
But in terms other than resolution comparing the D1x and D100 is like comparing the F5 and F80: if you want (or need) the facilities of a professional camera then you pay a hefty premium for it.