Sorry for my previous post - I didn't want to be mean and probably misunderstood you a bit. I thought about this trip to Himalayas in terms of a "lifetime adventure" - and that's why I thought it could be worth changing the gear.
Then I read your profile information and know the reason of that trip :) Sorry again.
So - if you're going to stay with your Nikon system I think that the Nikkor 12-24mm f/4G ED DX is really a great lens for you. You may be also interested in Sigma 18-50 f/2.8 EX DC - as your midrange zoom lens. I have this Sigma (Canon mount) and cannot say a bad word about it. Most of my panoramas were done using that lens -> http://www.photo.net/photodb/folder?folder_id=656901
A good friend of mine (and a great landscaper) have been using this compination for a long time. You may see some results here -> http://www.trekearth.com/members/lonelywolf/photos/Europe/Norway/page3.htm
I think someone has already posted links to those lenses tests - but don't rely entirely on MTF charts and technical information. Sometimes they make life even more difficult :)
Last, but not least - is a choice of proper graduated filters. I think this is the most important gear in landscape photography, and used with care can give amazing results.