Will you be north, south, east, or west of ATL? Direction determines access to particular races. Anyways, here are my suggestions:
1. Peachtree of course
2. Atlanta Marathon, 1/2 marathon, 5k- usually run around March each year.
3. Dirty Spokes puts on several trail races in the Fall
4. Atlanta 10 miler and 5K in October.
5. There are several 5k, 10k, and 1/2 marathon options on Thanksgiving.
ATC puts on open track meets weekly during the first half of Summer if you're around and interested. There's usually a couple decent runners in the distance events, but they're mostly for kids and community instead of competition.
ATC website is a good resource. You can also google "road races in ___________" and fill in whatever city you're in that will give you more local results.