how much are you sellin that D100 for?
The D40 isn't bad if you are using it as a starter and aren't planning on taking your photography too far. They are limited on lense use and are quite a bit smaller than your average slr, especially the D100. Thats a pro in comparison. If you are looking for a bit more I would recommend the D60 or D70. The D70 is the same megpix as the D40. If you are use to shooting Nikon stick with Nikon. Especially if you already have some lenses.