Acting wise, I think Maggie Smith will do a great job, but she is still older than I picture McG. I've got McGonagall pegged in her middle 50s; she's old enough to have taught the James Potter Gang, but I'm guessing she didn't come to Hogwarts until Dumbledore was promoted to headmaster (since he used to be the transfiguration prof.; I'm assuming he was a Prof. & probably head of Gryffindor until he was promoted, and she replaced him in his old job). We know that D. became headmaster sometime between Remus Lupin becoming a werewolf and turning eleven (because Lupin thought he wouldn't be allowed to go to Hogwarts, until D. took over).
Anyway, someone a ways back somewhere mentioned Diana Rigg as a choice for McG, and that's probably closest to what I've had in mind.