Thing is, what you're looking at is a wide angle lens.
And for a wide angle lens, the optimal thing to have is a full frame camera. Which the D90 is not.
The D700, D3 and D3x is.
However, that's not to worry; you can still use a wide angle lens on your camera, which has a crop factor, or focal length multiplier, of 1.5x.
You can choose from Nikkor, Sigma, Tokina and Tamron.
Tokina, just to note, is more expensive than Sigma or Tamron (usually), but the quality tends to be better because its made by ex-Nikon engineers.
Still, you won't pixel peep, I suspect (cropping to 100% and examining the photo very, very closely), so a Sigma 12-24mm would do.
In fact, Ken Rockwell can help:
http://www.kenrockwell.com/tech/digital-…
But don't really listen too much to what he has to say, he's a little cynical.
Do some research on those four; all are good, note!