I used to live in Grant Park, just south of the airport. I travel about 3 times a month so I know the airport VERY well and I would leave my house 75 minutes before departure and could walk onto the plane without breaking stride about 20 mins before takeoff.
BUT... to guarantee you make your flight I'd say you would have to leave the game at 90 mins before (Marta from the Dome to the airport involves a change AND takes about 30 mins from 5 points) OR with 6:00 to go to avoid any rush... whichever comes first.
My advice? Get a hotel and fly out the next day. Use Priceline and you should be able to get one by the airport cheap.
Thanks for everyone's help. I knew it would be close, but I thought leaving GA Dome at 8:00pm (2 hours before my 9:55 flight )would allow enough time to get to the airport. I will already be checked-in and have no bags. Would there be an advantage to walking to the 5 points station to avoid the crowd traveling North and the train change?