You will be fine with that one. I think that 40 degrees rating is for when you are wearing very little inside that sleeping bag. you can probably do a bit colder than that if you keep some of what you wear during the day on a night.

Another thing is that everyone's body work a bit differently. What is warm for me might not be "warm enough" for another person. 650 fill down has to be pretty warm tho <img src="/ubbthreads/images/graemlins/laugh.gif" alt="" />