Go for any of the mountain hardwear universe bags. They are down filled, very warm and expendable. If somebody who is going with you is a significant other you can zip two bags together. I think they have a +5 version for $250 and 0 version for $300. I slept in mine (0) on Kilimanjaro and Mt. Kenya and was pretty warm. If you really get cold you can always buy a fleece liner. My girl uses one since she is a mammal as opposed to me who is a reptile.

The most important thing besides warm sleeping bag that you have to remember is ground insulation. When I got my bag I took it out for a camping trip in May. I was freezing at night (60 degree weather) becasue I was not insulated from the ground. After I got a therma pad everything changed. You wouldn't believe how much heat you can loose while touching the ground.

Also I was always tempted to try some of the military bags but I have no reason to buy anything alse since my sleeping bag is so good. But I heard a lot of guys speakign very highly of them here.

Matt