Another word about MARTA when you're booking your flight, make sure you don't get a super early flight or you won't be able to take MARTA to the airport. The train shuts down around 2 and doesn't start running again until 5, so just make sure you have a plan to hang somewhere for those three hours if you plan on using MARTA to get around.
DragonCon can definitely be overwhelming, and I say that as someone who has lived in Atlanta for eight years. As the con nears and you find you have other questions specific to getting around the city, I'd be happy to offer a local's point of view. That goes for anyone, really, who may happen to read this thread.