I recently returned from a car camping trip where the low was 23 degrees. I used a Cabela's army cot, a 0 degree mummy down bag and a ThermaRest self-inflator. Now, I'm a cold sleeper, so the rating for the bag was more like a 25 degree bag. I slept warmly and comfortably.

I think what made the difference was what I wore while sleeping.
I had "expedition" weight thermal underwear, wool hiking socks and, most importantly, headwear. I brought a watch cap, a heavyweight balaclava and a lightweight balaclava. I brought these because I did not know what I would encounter- you never know as I was in the High Sierras. As it turned out, all I needed was the watch cap. With this set up, I didn't need to cinch up the hood to my bag.

The balaclavas cover pretty much your whole head. My heavyweight version covered my head with the exception of my eyes; there's flap that covers my nose and there's mesh over my mouth.

Having the insulation under you is very, very important as pointed out with other posts. Minimally, I suggest a closed cell pad (such as the Ridge Rest); but a self-inflator AND and closed cell would be ideal. I would put the closed cell on top of the self-inflator.

Winter camping is awesome!