Low light conditions are the most challenging conditions for any photographer. That is were the pros earn there salary. Equipment alone won't solve all of your problems. You need the corresponding skill to go along with it.

That being said, a large aperture prime lens is what you want. Variable zoom lenses are great for versatility, but will never produce as nice as a picture as a high quality fixed length prime lens. You probably want a 35mm f/2.0 lens.

Also, as Rick mentioned, since this is lightweight backpacking forum I would be remiss not too mention hybrid systems like the Nikon 1 and Panasonic micro 4/3rds. These are made for people who are novices that want to get more out of there cameras. They offer interchangeable lenses but without the mirror of a dslr. So they are smaller, lighter, and cheaper, but still offer the ability to put great glass in front of your camera. It is an emerging market, so the offerings are still pretty slim, but it is likely this area will take over entry level dslr market in the coming years.