You can add insulation to the floor without going the rafters - if you do put insulation under the roof you need to add ventilation chutes so you have adequate ventilation and moisture dosen't accumulate beneath the decking. As for insulation I like the treated paper product and there's a new thin roll-out product (solar sheild?) that also provides some R value. I'm leaning away from fiberglass.
I am in Atlanta too - I put in turbine ventilators and they work fine and require no electricity (I think they were about $20 a piece and come in black, brown or galvinized). Good Luck!