Remember that with the sensor size in D40/D40x, the effective focal length is 50% greater than the nominal focal length.
That said, the advice I've received on this board for my D40x is that a 200mm lens is more than enough for shooting wakeboard shots.
The two Nikon AF-S zoom lenses I looked at are the 55-200 (effectively 82-300) and the 18-200 (effectively 27-300). Both have vibration reduction, but that's not important when the object is moving such as in boarding shots.
The 18-200 is a little faster, but it costs a LOT more than the 55-200. Also, some owners have complained of lens creep, but not everyone. So, I got the 55-200. I can't wait to try it out.