Live somewhere near a Marta stop. Preferably on the east-west line (I would recommend towards the east, particularly towards the Decatur area, which is nice).
Parking downtown is a real PITA, so the fact that GSU has its own Marta stop is a big plus. By living on the east-west line you can avoid the Five Points station (which is the hub where you can switch from east-west to north-south), which is kinda icky sometimes.
If you're planning on driving every day, DO NOT live anywhere OTP (outside the perimeter - the perimeter is the I-285 beltline that goes around the city). Your commute will be horrid. I live in the Emory area (just outside of Decatur) and this summer I'm working just around the corner from GSU, and it takes me about 25-35 minutes to get down there. If you live OTP, you're starting to measure your travel time in terms of hours per day, and you're going to need that time to study.