The Hawks are the professional team of the city, and they have a solid following of fans. The stadium is at the CNN towers area; right in the middle of downtown. Admission is about $20-40 range, depending on the seats. Caution to you if you go to a game. Fans can, and do get "roudy" with beer infused, and you will feel like you are the "minority in the crowd" because you are.
I had the chance to see a NBA game Hawks versus Chicago Bulls the same day I arrived to Atlanta. Do not miss it. It is not only a basketball game, the whole time you feel like you are in a show, cheerleaders or other shows between half times... It was a pity the locals lost.
Atlanta has all the major professional sports teams - football, baseball, basketball and hockey.
The football Falcons play from the Georgia Dome. The basketball Hawks and the hockey Thrashers are played from the Phillips Arena next door to the Georgia Dome. Both the Dome and Phillips Arena are within walking distance of downtown Atlanta on a MARTA line. The Atlanta Braves play at Turner Field, about 2-3 miles southeast of downtown.
Being Canadian, and visiting during hockey season, we had to go to a couple of NHL games. The Atlanta Thrashers have been in the league five years now (in 2004), and the team has not reached post-season play yet, but they are getting better each year.