Hi Tuk, the look and feel of a camera is very important. Some just feel so right in your hands that you WANT to go and take photos with them! So I would always recommend doing what you've done, trying them in person, and being influenced by it...
The D80 is superior to the 400D / XTi in build and features, but it is also more expensive, so it boils down to budgets - and also which lenses you prefer. The Nikkor 18-200mm is certainly a great option which will cover you for almost all situations, capturing big landscapes at one moment, then zooming in to grab a wildlife or detail shot in another.
If you can afford a D80 and 18-200mm and don't mind the size and weight when trekking, then it's a great combo. Having one lens will also minimise the risk of dust entering your camera, but remember you'll need to keep it attached!
That said, the 400D plus 17-85mm is also a nice combination, and if you want something really small and aren't bothered about anti-shake, the Olympus E-410 twin lens kit is really sweet. Or the E-510 with built-in camera shake (albeit not as cute as the E-410!)
On a practical note, I've visited South America and it's a fantastic place but I've met or known many people who've had things stolen, so you'll want a camera you can keep on or very close to your person at all times - and discreetly too. Think carefully about this when making your choice. I personally lugged round a big DSLR and two lenses without any issues, but didn't leave them slung round my neck or in a dedicated camera bag, advertising my kit!
PS - thanks for price-checking using our links - it all helps! If you decide to buy from one of our affiliate stores, like Amazon, we can also receive a small commission. To do this, just click through to the shop from our affiliate stores page
. This works for books, CDs, DVDs or anything else you buy during that session! Cheers!