One thing not mentioned, somewhat controversial - vapor barrier liner for your bag.

Increases rating ~10f. Keeps bag from collecting moisture during periods of extended use.

Some of the ultralightweight backpacker crowd regularly sleep in their rainwear to save weight, close to same effect as using a vapor barrier liner. The practicality of this depends on the prevailing humidity and how much an individual perspires during sleep. I think this would work better in the western US, not so much in the east, in areas where nightime humidity is regularly 60%+.

http://www.campmor.com/webapp/wcs/st...0226&langId=-1

re above link. 10 oz of down will get your more insulation than 10 oz of coated nylon. if you can afford it. but maybe this would be a cheap fix to your problem, far cheaper than a new bag.