It depends on what you want to see and how much you want to see. I used to come here mostly for nightlife....see one or two interesting things during the day, go to dinner somewhere fun, maybe see a play, then stay out until 4 or 5 am.

Three days seems like a reasonable amount of time to me.
Some different types of destinations, depending on what you're interests are:
MLK Jr. Memorial and National Historic Site
Carter Center/Presidential Library and Museum
Fox Theater
High Museum
Georgia Aquarium
Centennial Olympic Park
Stone Mountain
Historic Downtown Atlanta/Fairlie-Poplar District
Margaret Mitchell House Museum
ZooAtlanta
Fernbank Museum of Natural History
Falcons, Hawks, Braves, or Thrashers game
Atlanta Cyclorama
Atlanta History Center
Atlanta Botanical Garden/Piedmont Park
Underground Atlanta
Atlantic Station, Lenox Square, Phipps Plaza
Center for Puppetry Arts