I have used the 18-70 DX quite extensively with the D70 and D200. I found this lens to be quite sharp, only below about 24mm I found the IQ somewhat lacking. So I got the excellent Tokina 12-24/4 specifically for landscapes. I am not familiar with the D80, but I believe its performance is close to that of the D200. The D200 is a very nice camera, except for its poor low light performance. If you don't need good high iso performance, I would keep the D80 for the time being and get an FX capable wide angle lens, either the 16-35/4 or the truly outstanding 14-24/2.8. After this, I would replace the 18-70 DX with the 24-120/4 VR or the very good 24-70/2.8. At this point you will be well prepared to consider buying an FX camera. If you acquire a D600 now, you will be severely limited by your lenses and changing to a D7000 would in my opinion just be an interim solution.