I read a lot of the posts here, but don't generally post that much. I've got to chime in here.
Fogo de Chao is an absolute must. Try to go at lunch. They serve the EXACT same cuts of meat at lunch as they do for dinner except for half the price. It is the BEST $22 worth of food money can buy in my opinion. The salad bar is beautiful but if you're a serious carnivore, skip it. Save that room for the meats. Oh yeah, their mashed potatoes and fried plantains that come with the meal are really good. The cheesy biscuits are good, too.
As for the Q, I've lived in or around every suburb of Atlanta for the last 20 years. I've been on the east side, west side, and north sides of Atlanta. The only BBQ I will spend my money on in Atlanta is at The Swallow at the Hollow in Roswell. It is worth the drive. To me, Fatt Matt's is decent and is more of a trendy, tourist type of place to go. Since you will be a tourist, it may be a good place for you. The Swallow at the Hollow has an affiliation with the Blue Bird Cafe in Tennessee as far as the singing talent goes. They have country 'n western shows on certain evenings for free.
If you like seafood and are in town on a Sunday, Ray's on the River has the most incredible all you can eat seafood brunch you will ever experience. Ray's is in Marietta and is located right on the Chattahoochee River. Ray's does not disappoint any day of the week, but the best value is Sundays or lunch-time during the week. Their sushi, ahi tuna, crab cakes, and lobster are sinful. If you're going to be downtown, they have a Ray's in the City which is the same restaurant except not with the ambience of the river.
If you like Asian food, I highly recommend Pacific Rim Bistro which is located downtown. They serve a combination menu of Vietnamese, Cantonese, Japanese, Chinese, and just about any other "ese" you can think of. Once again, prices are better at lunch.
If you want some good country home cookin', I would suggest Mary Mac's Tea Room. It is yet another tourist hot-spot, but this one lives up to its reputation a little better to me. It's not my grandmother's cooking, but it's the closest I've ever found in a restaurant. Their cracklin' cornbread is good.
All of the other Georgia boys who have chimed in are definitely trustworthy with their knowledge of Q'ing. I'm 6'1", 325lbs. You can trust me about knowing food. I didn't get my girlish figure by eating at bad restaurants.