If it's car camping, and you won't have to hike very much, then I have a suggestion:
Use a quilt. No not an underquilt, but a regular quilted quilt. I do this with my mayan hammock. I double the quilt up and put it under me, then I'll use a sleeping bag on top of me. I've done this many times and it keeps me cozy. However, I've only tried it down to about 24*F. My sleeping bag (cheapo-walmart one) is only rated to 32*F. I'll have to dig out my nice expensive sleeping bag if it gets colder.