Sleeping bag makers are slowly coming around to the idea that women need their own style of bag, but it's not a fast evolution. To some extent I see whysleeping bags have such a basic shape that reconfiguring it for women will add production costs without a huge benefit. Still, it's also true that a women-specific bag can be created. It should be cut a little narrower at the shoulder and wider at the hips, with maybe more insulation around the torso and feet. As a general rule, I think women should buy a warmer bag than a man for the same climate. In theory, most women don't sleep as warmly as men, largely due to their lower muscle mass.
Anyway, pretty much the leader in women's sleeping bags is Sierra Designs. Their Synthesis ($299) is a zero-degree women's bag that uses good 600-fill insulation. For more dough, the Katie Crux ($475) uses 775-fill down and has a zip-off top, so you can adjust it for different temperature conditions. And, although it's a men's bag, you might look at the Marmot Aguille ($499), a high-quality bag that's rated to minus five yet has a very slim cut.
You also could call Feathered Friends, tell them you want the Petrel (the ten-degree women's bag they makeit's $370) but want it over-filled for more warmth. They can do it for you, I know.