On a DX camera like the D90 a 20mm lens comes out to be equivellent to a 30mm lens which is not really that wide. I too would agree with taking a look at the Tokina 11-16mm f/2.8. The range of the zoom is fairly narrow for a zoom and the quality of it puts it into nearly "prime" class. No you do not need a converter for it- just be sure to get one with a Nikon mount. It is also available in Canon mount.
The longer the range you give a zoom, the more you compromise on the image quality. The 80- 200mm covers a very wide range and considering that it really does a remarkably nice job. If you take that same range and break it up among more lenses then each one will be trying to cover a smaller segment with a prime lens being the minimum range so it can be optomized to work best at that focal distance only. Since Tokina kept the range small, they can be more careful on optomizing the image over the entire range of the zoom.
There is a thread on here somewhere with some images people have taken with their 11-16mm lenses in the Sigma/ Tamron/ Tokina forum.
The longer the range you give a zoom, the more you compromise on the image quality. The 80- 200mm covers a very wide range and considering that it really does a remarkably nice job. If you take that same range and break it up among more lenses then each one will be trying to cover a smaller segment with a prime lens being the minimum range so it can be optomized to work best at that focal distance only. Since Tokina kept the range small, they can be more careful on optomizing the image over the entire range of the zoom.
There is a thread on here somewhere with some images people have taken with their 11-16mm lenses in the Sigma/ Tamron/ Tokina forum.