I would recommend a bag in the 4 lb range that compresses well and is rated down to 0 degrees. To achieve that you basically have 2 options that I know of. Either one of the new synthetic down synthetics such as Polarguard 3-D OR go ahead and spend the money for a down bag with a goretex type exterior shell that sheds water. The major problem with the synthetics is that they will go flat quicker than Down. MUCH QUICKER. The way to extend the life of the synthetics is not to ever leave them in a compressed state except when packing in. I hang mine in a closet most of the year, travel with them in a big "storage sack" that usually comes with them, and put them in a compression sack only when I get to the trailhead and start my pack in. When I arrive at a backpack camp, I take the bag out as soon as I have my shelter up. If aren't careful, and leave them in a stuff sack for an extended period, they will lose their loft/warmth.

On the other hand, I usually try and treat Down similar, but have never had a probem with it going flat, even when I forget and leave it in the stuff sack for extended periods. With the new, Goretex type outer shell, it should be waterproof unless you drop it in a creek. My brother has one and hasn't had a problem with moisture, although we have never used it but in a tent.