While in the Army, I was lucky enough to work in units that actually understood cold weather operations. We has North Face Cat's Meow bags with lightweight bivi bags for temperate operations and a NF warmer bag (Tangerine dream? Can't recall the name) for really cold weather. I've spent nights out in the open and under poncho hootches in temps below -40 F, and despite my best efforts, froze my ass off. Later, we got Wiggy's double bags, but still when it's really cold, you're gonna get..... Out of the cold and into a shelter or snow cave (If there's any friggin' usable snow).
So much for me sniveling about days gone by- OP, check Sierra Trading Post for a fair selection of bags at better than retail prices and don't forget a ground pad or at least piled up leaves or grass underneath. If you're tall, be sure to get a tall bag. Your feet will get cold if they're pressed into the bottom of the bag. Stick with synthetic fill. Down's a deathtrap if you get stuck out and it gets wet.