There's an active thread about Tampa to Atlanta, and that has several recommendations for the first chunk of your trip. You'll be taking I-75 through Atlanta and north to Marietta and then I-575 up to Ellijay. Marietta is full of criminally underrated restaurants. My blog, Marie Let's Eat!, has a pretty detailed section on Marietta, on account of us living here (if it's not too tacky to hawk myself): http://marieletseat.blogs...nesaw%20%2F%20marietta
Notably absent is the Roadfood-approved Williamson Bros. BBQ. This is because the site is, IMO, quite mistaken. That is some terrible, terrible barbecue.
The I-575 corridor is really not represented well among barbecue talk, but there are some very neat finds, including Mo-Ribs in Canton, Two Brothers in Ball Ground, Bub-Ba-Q in Woodstock and the original in Jasper, and, of course, Col. Poole's in Ellijay itself. Massively improved over the years, this is among my nine or ten favorite plates of chopped pork anywhere, and an absolute spectacle.
Other than BBQ, there is an old-fashioned Jiffy Freeze in Canton, similar to the famous Mrs. Story's in Opelika, AL. In Woodstock, you'll find excellent pizza at Vingenzo's (mentioned in the other thread) and pretty good burgers at Canyon, a wannabe chain getting started.