I just purchased my first digital SLR, a Nikon D40. This will be my first foray in the realm of SLR photography. I'd like to purchase another lens in addition to the 18-55mm that comes with the D40 kit. Over the summer, I'm going to be traveling around South America taking photos and such. My principle shots will be of landscapes and people, mostly outdoors. I'm flexible in terms of cost, but would like to keep things on the cheaper side since I'm just a beginner. Any recommendations?