Does she usually get car sick? If she handle's cars and trains without a problem, she may be okay on a plane.
If she's okay during take-off, she'll probably be fine for most of the flight, except for the landing.
Whether you give her dramamine or not, try to have her do activites that won't make her use her eyes too much. What makes people motion sick are disturbances in their balance sensors (not the correct medical term, but it fits here). Reading, coloring, drawing, and other activites involving a lot of looking around, which uses these same sensors, so on the plane, try not to over-stimulate them. Maybe have her listen to music on headphones, read her a story, or just have her play with her favorite stuffed animal or doll.