James I too shouted myself a new Nikon recently but I got a D5100, great camera and for the track ive added a Nikor 55-300 tele lens, I find it great if you can get to within say 25-30 meters of the action, as you can then blow them up and crop down to get a good shot, a 500mm would be nice but maybe later.
cheers John
PS don't go near a 2x convertor, their crap.:furious: