The IT Guy: I imagine the difference between the ability to use various lenses in the D7000, I say this because I come from a D40 also and I had to learn the subject of the lens with autofocus and built-in motors.
is it true that the D3100 Nikon need integrated built in motors lenses for autofocus?
Yeah, the D40 definitely drove my lens purchase decisions because I had to stay with AF-S lenses in order to have AF. Now I have the freedom to explore other lens options and still have AF. Although, my next lens will likely be a macro (don't have one yet) and AF is way less important for macro work where you probably want to manual focus anyway (at least when using a macro lens for what it's intended for, close-up photography). But I do like the freedom of having an AF motor in the camera body.
I believe the D3100 is the same as D40. It's their small body, entry level DSLR, so I believe no AF motor.