I have gone to the Atlanta Race the last 3 years. The 2005 March race was the most crowed I had ever seen. Seemed like almost all camping areas were full. I usually stay in the "Handicapped" campground area off Turn 2 (gate 10) and even it was almost full this year; only a space or two left. Rates are reasonable there, but, at least someone in your party had to have a documented disability.
In the future, go to the Atlanta Speedway's website.
http://www.atlantamotorspeedway.com/
They have camping info. there. They have both reserved and unreserved areas. You didn't say what kind of camping rig you have. They have separate areas for motorhomes and tent/tent campers.
I would have posted this in time, but I just joined post March race.
http://www.atlantamotorspeedway.com/<pre class="ip-ubbcode-code-pre"> </pre>
Good Luck!