If you're seriously determined about going for an MBA - I'd say go kill the GMAT now. When you start working, your ability to solve mathematical puzzles will degrade; you will never be as smart an flexible as you are during undergrad years. So if you have a lot of time - I think it's worthwhile to do a 1-2 month hardcore preparation and go get a 780 or so.
After all, if you end up not going for an MBA - well, you lose $250. Big deal. The satisfaction and pride you get from acing the GMAT will be with you anyway