There is a fantastic, dedicated mountain bike trail network maintained by Sorba just north of Atlanta. It's called Blankets Creek and consists of about 15 miles of singletrack in three loops. Lots of log crossings, creeks, and even a skinny on the toughest trail.

From Atlanta, take 575 north to exit 11 (Sixes Road). Take a left back over the freeway and go down Sixes road about 1.8 miles. The parking area is on the left. You can read more about it here: http://www.sorbawoodstock.org/trails.html