Wow, Thanks for all the help.
I think for my needs (North-East winter Backpacking and eventually some mountaineering)I will go with an overbag and a 0 or -10 down sleeping bag.
Other then MEC's overbags, any other companies to look at? I live in the States and would like to avoid the 18.00+ shipping from Canada.
Also, Most of my trips are a week or less long (backpacking), do I need to even worry about bringing an overbag? My concern is the extra space my down and then an overbag will take up.