I have been considering the Tokina 12-24mm and the Sigma 10-20 for my D80. although i like the build quality of the Tokina I think i leaning towards the Sigma, I want less overlap and more wide angle!
the Nikon is supposed to be the best option for Nikon specific lenses in this category but its also almost double the price and isn't noticeably better.
I say go with the 10-20!
A good comparison of Wide angle Zooms
Aside form the 10-20, the 18-200 VR is a great lens but if you don't have the money for that and want to replace your 18-50 you could always upgrade to a 18-55VR