No, you can't be a sith. Darksider, yes, but not Sith since there are only two at any given time, and right now, those two happen to be the Emperor and Darth Vader.
This game has never been about playing a Jedi. It's for all those other occupations that get dismissed whenever people have the chance at being a Force-user. If you want to play a Jedi, go for Outcast or Academy or any of a dozen other games.
Besides, being a Jedi is supposed to be a hard thing that only a few people ever actually attain. I'm glad Galaxies has kept that spirit, and even made it unhealthy to be a Jedi.