If you can, go for the down bags. Even the most exotic synthetic bags have a tendency to feel a tad clammy and stuffy. They also don't last as long and are more prone to developing cold spots over time. Unless you're around lots of moisture (kayaking, rafting, rain forest travel, etc) down rules.
The previous poster was dead on about women's bags- well worth the money. Also realize that all down is not alike. A feather is not just a feather. You want to be aware of "fill power." The higher the number the higher the quality of down. We only use 775 or 800 fill bags because they feel like their just full of warm air. A 600 fill bag can feel like it's full of pokey, sharp, lumpy feathers.
When picking a temperature, realize that a 20 degree bag will allow you to sleep in 20 degree weather but...it's going to suck. My wife and I have at least a dozen bags and we always grab the bag that is 20 degrees colder than the low temps we expect.
Lastly, don't forget to get some nice PJs. Seriously, the duds you wear in a bag are critical to your comfort. Only wear thin technical fabrics like Patagonia Capaline. They serve two purposes. They keep your bare skin (and yukky oils) off of your neat bag. The ding dong that said, "sleep naked" must have owned some oily, stinky bags. Techical long underware also helps transfer your body heat to the bag baffles. If you dress too warmly, your close insulate you and not the fancy bag. A good hat is helpful. I'll kill my tentmate if he has a nicer pillow than me! A good pillow is key. The same is true for some sleepers with sleeping pads.
Sierra Designs, Marmot, Big Agnes and others are my top picks. Get her set up well and she'll be loving it. There's so much more to it than just the shape of the bag.