Sigma offers a 135-400mm f/4.5-5.6 APO zoom lens that would fit the bill fairly well.
B&H currently has it listed for $589. They also have an 80-400mm f/4.5-5.6 with built in image stabilization, but that one goes for $999, which may be more than you're looking to spend.
Tokina also offers an 80-400mm f/4.5-5.6 lens that retails for $649.95.
Sigma and Tamron both also offer long zooms that go out to 500mm with a maximum aperture of 6.3 zoomed all the way out, although both are a bit more expensive. The Tamron 200-500mm f/5-6.3 goes for $879, while Sigma offers two long lenses within reach of mortal man: a 50-500mm f/4-6.3 for $999, and a 170-500mm f/5-6.3 APO zoom for $769. For those with money burning a hole in their pockets, Sigma also has a 300-800mm f/5.6 APO zoom lens that can be had for the princely sum of $6,999. If you're interested in Nikon glass, they have several prime super-telephoto lenses available for what would amount to a down payment on a starter home.
Personally, I'd probably go for the 135-400mm Sigma lens. It's the least expensive, but it has a wide zoom range, and it's also an APO lens, and carries the DG designation, which gives the best correction for various kinds of aberrations.