Firstly the D1X doesn't have a full size (35mm) sensor it's a 1.5 crop like the others. It's also getting pretty old now so unless you really need what it specifically offers (i.e. build quality) I would look elsewhere. Cameras have come on a long way in the last ten years, image quality wise I would expect it to be blown out of the water by the current entry level models. By image quality I mean resolution, sharpness, colour reproduction and noise.

Same applies to the D100 to an extent. You'd be surprised by what basic features are missing from even pro level bodies that are a certain age. I'm not a Nikon shooter so hopefully someone else can give you some other alternatives with regard to lens compatibility, but my advice would be to look for something released in the last four years.

I know you're only doing this as a hobby and so maybe image quality doesn't matter, however I speak from experience when I recommend you get the newest body you can. I recently upgraded from a 1D mkII to a mkIV, a gap of about 5 years in terms of camera developments. Instantly I found the new body is just so much nicer to use, the controls are laid out better and more logically, there are loads of useful settings that were missing before. The screen is so much better I don't know how I coped with the tiny low res screen on the mkII. Battery life is also better, partially because of newer technology and partially because the original battery is worn out.

At the end of the day a body that is pleasant and fun to use is the important thing. If the new body is a pain to use compared to your D40, you wont look forward to using it and wont get good pictures.