[Dabhand16]

I don't think you will miss the 15mm between 55 and 70. Think about the photographers that only use prime lenses. They will have 'gaps' between all of their lenses, and some 'gaps' might be as much as (or even more than) 100mm. It is not necessary to have a continuous range of focal lengths. I have cameras that I used to use years ago that have not only fixed focal length lenses, but they are not interchangeable either. So I had to use one lens with one focal length for everything.

Any Nikon mount lens will fit. The D40 is different to other Nikons in its focussing system. Because of this, there are not so many lenses to choose from that will auto focus.

Nikon do two 70 - 300 lenses one is a VR, I don't know if either will be compatible with the D40 focussing system. The Sigma is worth checking out too.