Thanks Michael,
There's definately some good recommendations already in this thread. I used to be of the "prime lens only" fraternity, but I bought an 80-400 VR recently, and it's great for travelling with, although the focus moves at glacial speed 
Zooms have really come a long way....
The 70-200 VR is a better lens (and more expensive). At F2.8 you could put a teleconvertor on this and it will still yield great results. It also has a silent wave motor and a better implementation of Nikon's Vibration Reduction (the 80-400 was one of their first attempts and whilst good, it has been refined since)
As for primes in this budget range - the 300 F4 is an excellent lens, and for this sort of money, you could buy a 1.4 teleconvertor to go with it.
But that's just the Nikon offerings - you'll often get more for your money (and sometimes better lenses) by looking at the other Nikon-compatible lens manufacterers
There's a few of these lenses reviewed on Thom Hogan's site : http://www.bythom.com/
Happy shopping!
Steve