I think it's useful to emphasize that technique matters more than equipment, particularly in this context. For example, an inexpensive consumer zoom plus a decent tripod used well will get you better images than an expensive pro lens with no tripod or poor technique.

A similar high-level point: buy equipment that addresses your specific needs. In other words, to justify and plan an equipment purchase:

1. Identify specific shortcomings in your photography you'd like so solve.
2. If you can identify specific equipment acquisition that will help you address these shortcomings, and you understand why, go for it.
3. If you can't, or things seem vague, don't buy anything yet.

In other words, buy equipment when you can say Purchasing X will improve my Y by helping with A, B, and C. If you can't fill in the variables with specificity, the equipment purchase is not justified from a photographic standpoint.

(To be clear, the research and purchase of equipment, and great interest in same, is a valid variation on the hobby. But make this choice consciously.)

Take a read through Thom Hogan's site and reviews, particularly his articles "Rationalizing Lenses", "Nikon Digital SLR Lens Kit Recommendations", and "Tripods 101".

Random tips:

• If you are actually going to carry two camera bodies, don't bother with a superzoom. (Though, I'm skeptical of dragging around two bodies when out and about for landscapes.) Carry two lenses with complementary focal lengths. This might be a good plan even if you're only carrying one body.
• The 16-85mm is also well regarded.
• I do have the 18-200 and like it, though I'm not 100% convinced I'd buy it again, since I often want to go a little wider.
• Consider something strange, like the 10.5mm fisheye.
• At the risk of repetitiveness, consider a decent tripod instead of a lens as your next investment. The lenses you have will all take great landscapes used well.
• Don't hesitate to buy used lenses. I've purchased from and can recommend KEH; don't know if they ship to your side of the pond or if it's cost-effective to do so.