It doesn't depend on the camera body, it depends on the size of the lens. DON'T confused size of lens for focal length. If you can't find the specs for your lens, just measure it.
Yes
All lenses with Nikon mount are compatible with Nikon D40. If the question rephrase : "What lenses are able to autofocus with Nikon D40", then the answer is : the new Nikon AF-S lenses (with Silent Wave Motor inside), Sigma lens bearing HSM code, and upcoming Tamron lenses (18-200mm, 17-50mm and 70-300mm).
Yes, it will fit. However, you will still need to see if the lens you have has or does not have features like autofocus or CPU connectors that will carry all the info that your camera needs... If it is a Nikon Lens it will work fine. So will others like Sigma, Tamron and Tokina. Carl Zeiss may not give you all the functions but will work in manual mode.
Yes
If you've been using the D40 for basic photography or anything below professional photography or photojournalism, I'd go for the D90. On that note, if you've been using a Nikon, I'd keep using it, a) because the lenses you have now will still fit, and b) the controls and menu functions are similar. Anyway, the 90 has a better "highest quality" setting than the new 40's, 3000's, or 5000's. It also has a larger display, and the ergonomics are fixed.
Yes
yes fo shoooW
I think so
yes, sure can
yes
A Nikon EM has a standard Nikon mount, so you can use any lens that'll fit on a Nikon, except for a Nikon Series G lens, on an EM. A Series G will fit but it won't work because there's no aperture ring.
nope