JerseyGirl, sounds like you need the Marie, Let's Eat! map, although I'm sorry that we've not gone west of Memphis: http://www.communitywalk.com/marie_lets_eat/map/939046
As for a couple of specifics to your route: Roswell is home to Spiced Right Ribhouse, which is very good, and the Swallow at the Hollow, which I really don't like, and a Dreamland franchise, which is doable for those who've never been to the original. Outside of BBQ, it's also home to the Fickle Pickle, which has amazingly awesome fried green tomato sandwiches.
Silver Skillet and Mary Mac's are fairly standard "when in Atlanta" replies. They're also about an hour from Roswell.
Personal bugbear: If you're only going to do one BBQ place in the Atlanta region, DON'T pick Fox Bros. Everybody does that. All these blasted road trip blogs go to twenty places in NC and five in SC, then Fox Bros, then Memphis. Drives me mad. It's good, and if you are doing four or five here, go. Otherwise, I recommend Heirloom Market, Sweet Auburn, Community Q, or Dave Poe's.
As for the road to Nashville, I-75 between Marietta and Chattanooga is absolutely packed with easily interstate-accessible BBQ, most of it quite good. I-24 between Chatta and Nashville is not. You can find really good stuff, like Martin's, if you're willing to take the detour.
Don't make the mistake that Chattanooga is a skippable bump on the road. That city is a foodie paradise. There is all kinds of terrific eating and drinking there. Shuford's Smokehouse is my favorite BBQ, but there's everything from an amazing brewpub (Terminal) to fabulous hand-mixed, all-natural sodas (Pure Sodaworks) to some of the best burritos I've ever had (Mojo).
Nashville: if the wait at Prince's is too long, try Pepperfire, but do get some hot chicken. I like Judge Bean's BBQ in Brentwood best, and also Hog Heaven in Centennial Park.
Memphis: People come to blows, but my three favorites of the criminally few I've tried are Leonard's (dry ribs and mustard slaw), Bar-B-Q Shop, and Central.