Extension tubes with Nikon get kind of wierd... As I remember only their shortest (11mm?) allows any kind of automation because it can pass electronic info from the lens to the body (the longer ones don't have contacts). I think Tokina/Hoya has a set that will do this though - that might be the one to get. A set of theirs will be about half of your budget.
I've only used tubes with a Bronica MF system, and that's been awhile - anyone else?