Well if your going to buy one now, the d70 is a better choice.
The d40 may be compatible with a fisheye but it won't autofocus it since the d40 doesn't support all functions (autofocus!) of AF lenses, only af-s. The d70 will.
D70 will shoot 3fps and the d40 can only shoot 2.5fps
Same size censor (6.1 MP)
Aside from the autofocus motor, the d40 does have a larger lcd so its up to you.
If you have any more questions I could help you out.
Since these are both about the same price these days I'd rock the d70.