I was born and raised in Atlanta, and am very familiar with the traffic flow.
If you're coming down I-75 from the north, don't plan on approaching Atlanta and the perimeter highway, I-285, before 10 AM, traffic can be heavy on I-75 as you approach I-285. The same is true if you're approaching from the south.
You want to avoid morning and evening rush hours. Morning rush hours can last from 6 AM until after 9 AM, and in the evening from about 3:30 PM to almost 7 PM. The poster that mentioned going through town at night has a pretty good idea, but a painless daytime transit is not out of the question even on a work day.
If it were me, I'd avoid I-285 and stay on I-75 through town, in general it's shorter and quicker if there's no heavy traffic. However, this road is susceptible to wrecks and traffic at any time of the day or night so although the odds are in your favor for a good run through town, anything can happen. If you opt for I-285, you'll have a lesser chance of heavy traffic but you'll have to travel a few more miles.