You have received excellent advice. Lenses are more NB than camera bodies...
1. My wife and I each have a D7000, which really is a very good camera, and very cheap today. If you upgrade from D90, rather take 2 steps and go for the D71000 for the 1.5x DX factor and bigger sensor...
2. I lusted for the FX format and got one of the first D600 cameras. Seriously, very good quality, but disappointing for wildlife, cause your lenses feel insufficient with not enough reach for wildlife...
1. I had 2x 70-200mm f2.8 VR1 models -- the first was brilliant, the second not so good.
2. I now have the amazing little Nikon 70-200mm f4. It is seriously sharp, lightweight but solid, and works brilliantly with 1.4x tc, even wide open. Price = half of the 2.8 model.
3. Had a 300mm f4 -- brilliant lens, but AF packed up 11 months after buying it. Amazing close focusing ability too. Struggled to get sharp pictures on tripod -- not very good tripod mount. I also missed the versatility of a zoom lens.
4. Got a used Nikon 70-300mm VR for gorillas in DRC, but big mistiake -- NOT very sharp wide open @ 300mm! Need to stop down at least 1 f-stop for better results.
5. I do use a 200-400mm f4 VR1 -- get amazing results with it, but price!!! (got it cheap used from a friend). If you are serious about good quality, then try to get a NEW model 80-400mm -- seems to be highly rated by guys like Tom Hogan etc.
I don't use the 18-200mm any more, not good enough for newer sensors. Only use 24-85 for the D600 landscapes, it is OK.
The surprise for me was D600 plus 70-200mm f4!! It works as a "landscape" wildlife type of lens, with stunning results.
I mostly use the bigger lens, even with 1.4x tc on the D7000 for more reach.
Last point: with something like the D90 I suspect you shoot no higher than about 400 ISO??? With the D7000 you can get amazing results in LOW light (before sunrise, after sunset) up to 3200 ISO (over expose by +1/3 f-stop to limit digital noise)....
With the D600 you can go up to 6400 ISO when desperate, so really, you don't need a f2.8 so much any more...
Hope you get what you need.
Friedrich von Hörsten
PS. Don't be afraid to go for used equipment, as in Outdoor Photography etc. You can get very good equipment with very low mileage for decent prices.
Check some of my pictures on Facebook: Friedrich von Horsten Photography for the D600 + 70-200mm f4 combination.
``God, I can push the grass apart and lay my finger on your heart'' -- E. St V Millay