Understanding transgendered people?

[mizzrandom]

My really good friend has been steadily dropping more and more obvious hints that she wants to be/ feels like a guy. Of course, this could definitely be me reading too much into things, but just in case she really IS transgendered and is planning to tell me in the future, I'd like to be prepared for it and not blurt out rude/insensitive stuff. (Besides, nothing wrong with new knowledge)

When she came out to me as a lesbian, I just said OK and and moved on (well, besides switching boyfriend for girlfriend in discussions). Should I do the same now? Or make a big deal about it and, I donno, encourage her, thank her, or something? Should I be serious or be lighthearted about her coming out? What about pronouns (he, she, ze??)? Can I ask if she has a male name? Should I treat her like a guy afterwards, like stop talking to her about periods, my new bra, etc.? And if she's a transgendered guy with a girlfriend, does that make her gay or straight??

Lastly, is a transgendered lifestyle heathy? I think I heard somewhere that it's a mental disorder?

NOTE: The only experience I have with transgendered people are on movies and TV shows of drag queens or prostitutes. Not the best source, yeah? So if I'm being rude/bigoted, please don't just say I'm rude/bigoted, but on which statement/question is rude/bigoted and how I should correct it, OK? HELP MEEEE~~~ *flails*