I would also recommend looking at the REI Zephyr 20 bag. It's a synthetic bag that isn't cut too narrow like some other bags do in order to cut down on weight (like...almost every North Face bag I got in at REI). It's very warm, in fact I was hot in it at 40F and had to remove the separate mummy liner I bought and stripped down to my boxers and a thin pair of socks (and I was STILL a bit toasty.) By morning the temp had dropped to 32F or so and I was comfortable.

I bought it at REI in late December for $75 on sale only to see it for $40 at the January sale. They wouldn't price match it for me...I'm tempted to use the return policy to get my money back out of spite. I can understand if it's >60 days later or so and not matching, but within a month getting burned by almost 50% kinda stings. It's no longer online and I doubt stores will still have it, but you might find it dirt cheap on ebay if someone bought a few at that $40 price.