Are you thinking midwinter in NH? It gets way colder than 0. Even here in n. mass. it gets to -10 or -20.
For midwinter in NH, even a 0 degree bag is too optimistic. A 20 degree bag supplemented with clothes, liner and bivy wouldn't do it for me at below 0. With the liner and bivy, you could be adding 1 lb to 1.5 lb. Something like the Northface Tundra (-20F, 3lb 14oz) or Darkstar (-40, 5lb 15oz)synthetic bags might work; the big disadvantage is not the weight but the volume (Darkstar stuffed size: 12"x25" is 2826 cu inches tundra ~1500 cu in.).
For shoulder season (i.e. now, Dec. 1, not Jan. 1), a 20F bag supplemented with clothing might work.
For summer or warm spring and fall nights the winter bags (rated below 20) will be too warm and a 20 bag is usable.
For myself, I have a good 20F Feathered Friends bag, overstuffed with a couple extra oz of down -- maybe good to 10 or 15. I'm thinking of sewing a Climashield overbag to bring it down to -20. Mine is pretty tight for using a liner or clothes inside. I'm also considering the NF Tundra.
Campmor has Campmor brand down sleeping bags for 0 and 20 that are bargains