One Flew South is definitely worth it if you have the time, it's really the only sit down restaurant that's not a chain. All of the concourses are connected on the airside by the PlaneTrain or walking if you want to stretch your legs. You'll clear immigration and customs and then go through regular TSA security back into the airside. In my experience a lot of the RDU flights go from A or B, but over the last couple years I've had flights depart to RDU from every concourse except D, just confirm your gate when you arrive. I wouldn't worry too much about what gate you arrive and where you depart from, with the train it never takes very long to get where you need to be.