Just got back from the high Sierra, where we had wind, snow and craziness in August (great summer weather, yes?). I took a Jacks R Better down quilt that weighs 20 oz. and packs to the size of a cantalope, and a Neo Air medium at 13 oz. Was very warm both in my hammock and on the ground. The quilt is accurately rated to 25F, I believe. I have repeatedly been warmer than warm in this to below freezing.

I would mention that it is very worth seeking a higher fill power of down as well, as the huuuuuuge down sleeping bags you will find on the rack at REI are mostly 650fp, and my 800fp quilt compresses better and is in some cases warmer. Feathered Friends, Western Mountaineering, Montbell and Marmot have some higher quality bags that will last you decades with proper care.

You will find that sleeping bags are heavy and cheap, or light and expensive, but never warm and light and cheap. My quilt at about 250 (3 season, they have 4 season as well) was the best deal I found, and as I hammock it's really exactly what I needed. I have recommended the Ray Way quilts to people as they are also good quality but synthetic and come as a kit to make it yourself; for less than 100 you can have a very warm solution that while bulky is still less so than many sleeping bags. They also have two person quilt patterns.