As far as how different cameras work, have a look here in my Nikonians gallery. I've posted two files, full size, processed identically(*). They're both shot from the same tripod position, with the same lens, and the same subject, just a minute or two apart. The lens is the 200/f4 AFD Micro-Nikkor, one of Nikon's best, meaning that it's far and away capable of extracting the most out of either of these two sensors. Indeed it probably out-resolves even the 24mp D3x.
One of those files was shot on the 4mp D2h, the other on a 12mp D2x. Eve looking at the full-size renderings at 100% zoom on a big 27" display, I think it's pretty hard to pick out the differences. It's not impossible, and I'm sure I could find some that highlight the differences rather than being indifferent, although this is the only direct comparison I have on file.
------ (*) Close inspection of the EXIF data will show that there is a slight difference in exposure, 1/15th vs 1/20th sec. This is due to the fact that I had a KatzEye focusing screen in one but not the other, with slightly different results. I added compensating exposure difference in processing, since this is not really about the difference as most would see it. (Ie most users would have a KE in either both cameras or more likely neither.)
_____ Brian... a bicoastal Nikonian and Team Member
My gallery is online. Comments and critique welcomed any time!