It really depends on where and what kind of planes you will be photographing. I have a D40 and the Nikkor 70-300mm VR
, and love it for plane spotting, but obviously my 18-105 would be better suited for static displays at an airshow and the like.
So if you need something for distance, then I would really recommend the 70-300 VR
, worth every penny so far. I don't know if you already have a general purpose lens, but the 18-105 is great for general stuff not to far away.
But perhaps the most important factor is how much you can/are willing to spend. We can recommend a killer lens, but if you can only afford something for 1/3 of the price then our advice is pretty useless. Let me know if you need anything else, and I'm sure fellow photographers here will also help you out.