I can't believe no one has mentioned McKinnons. It's right in the middle of Buckhead. I really don't know how authentic it is (though their website claims they are) since cajun really isn't my thing but you can at least take a look at the menu and see if it's what you're looking for.
http://www.mckinnons.com/
As for the one on Peachtree near the Amtrak station, that was Huey's. It's been closed for at least a year if not two.