While your decision on if you make Revan light or dark as well as male or female does have some minor impact on the storyline (some cutscenes mostly) it doesn't influence your light / dark alignment.
That one only depends on your actions and if you are light or darkside once you reach level 15. If you talk to Kreia then she will let you choose a prestige class to advance in. If you are lightside you get to choose from Jedi prestige classes while if you are darkside you get to choose from Sith prestige classes.
That kinda is the ultimate decision between going light or dark in the game.
