The Nikon D3000 has taken the place of the D40 in the Nikon lineup. There is not a lot of differences, but there are a few. I believe that the D3000 comes with an 18-55mm/f3.5-5.6 AF-S VR (Vibration Reduction) lens, where as the D40 comes with the same focal length, but no VR (I think). The price is almost exactly the same. Photo.net did a pretty decent write up on the D3000 when it came out. http://photo.net/equipment/nikon/D3000/review/
If you want to save money I suggest cruising craigslist for cheap bodies and lenses. I've seen D90s with 3 or 4 decent lenses used for around the same price, or less, than a D3000/D40 new. Something to take in to account here is size. The higher end Nikons tend to have larger bodies which will take up more room. It's only a few inches so not a big deal, but some people want the camera to be pretty small.
When it comes to picking between Canon and Nikon it really comes down to lenses. I chose Nikon because most of my friends shot with Nikon and I knew they would let me borrow a lenses when I'm not on the road.
Greg's suggestion about an 18-200 seems pretty good. That's a good amount of focal range to play with while traveling. I also must stand by the FAST memory card. This is an area where speed really does come in to play. When you want those back to back shots which come in so incredibly handy while traveling, you don't want to waste 10 seconds waiting for your memory card to catch up.
If you want to learn more about photography there is an area of reddit that is made just for this. I find it to be one of the better introduction to photography courses I've found online. You can find it here http://www.reddit.com/r/photoclass