Which Lense

[MarkLG]

Extenders: a word of caution regarding extenders on slower lenses like the 18-200's which have been mention here. If you start with an f5.6 lens then you'll lose 1 or 2 more f-stops when you add the extender depending on the magnification. You could end up with a lens combination which does let enough light through for the AF to function on some cameras, as well as reducing the usable shutter speed where you need it the most - at the telephoto end.
They're popular with pro's using highend lenses, which are usually f2.8 or f4 and can handle the loss of an f-stop or two.

I used the Nikon AF-S 18-70 on my D50 for my last trip, and was very satisfied with the results, but there were occasions where a longer lens would have been useful. I've been looking at the Sigma 18-200's as an alternative, but I'll need to give one a thorough test before the trip to check the quality thoroughly.