I have a Nikon D60 and a 55 - 300mm macro lens which I use to shoot planes (okay you are not going to use macro with planes). The fast planes are hard to catch and that is where the autofocus really makes it worth while. I wouldn't however settle for any lens which isn't atleast 300mm. I think 500mm would be the best though. I'm not to sure if you get VR lenses at that size but if you do, then I would have a look into that aswell. It gives you a much better shot with moving objects.