In the early summer and early fall I use a 20 degree bag. In the heat of the summer I use a 40 degree bag. I spend a lot of time hanging out in Baxter park, and it's usually a bit colder there. This february I slept on Chimney Pond in a 20 degree bag with my z-rest and down coat. The only thing that got cold was my hands, which were in fleece gloves. In the middle of the night I took the gloves off and stuck my hands in the pockets of my down coat and they were much warmer.
Last summer I bought a 40 degree lightweight bag at REI for 100 bucks. It weighs 1.4 lbs, no hood, no zipper. It works out fine during the warmer days.