I think there are 4 different ways to say the word.
N-gga: Talking about a homie, bro, etc etc. Generally okay for most to say.
N-gger: Talking about above or a black person in a more offensive way, but not to hurt feelings, like the difference between what the heck and what the hell. Easier to piss someone off with it, nonetheless, thus not advised for a white person to say just cause it may offend people.
N-gga (derogatory): Referring directly at a black person to say they fit a stereotype or are bad in some form. It's a pretty big no-no for anyone who isn't a gangster.
N-gger (derogatory): You do this if you're white and you in danger, girl.