I think first and foremost you need to decide how much length you need. If you could get away with only 200mm, a 70-200 is the obvious lens. Sigma makes an unstabilised one if budget is an issue for half the price. F2.8 is more important than stabilisation in my opinion, but thats another story for another day. 200mm should be fine for motorsport, but for wildlife you may want longer.
If you do feel like you need longer, then I guess the 70-300 or 80-400 are the next obvious ports of call, as are their equivalent Sigmas. Not really a fan of the 70-300 as its the same price as the aforementioned Sigma 70-200. but if you need the length you need the length. The 80-400 is a decent lens, although the focussing can be a little slow, we'll be due an AF-S upgrade in the next couple of years I guess.
If you feel like you may be able to get away with a fixed focal length, then you should look at the 300mm f4. I owned the AF-S one a while back and used it with my 70-200. The focussing is very quick, all be it not quite as fast as the 70-200. The size is great though, which is why I like primes, its smaller and much lighter than a 70-200.
Nikon D5000 and D300 with 12-24mm, 35mm f1.8, 50mm f1.4, 60mm f2.8 Macro, 18-200mm, 17-55mm f2.8 (all Nikon)