I'm sure this will not be something your wife will want to hear, but "transgender" is a term that has been used to just gather all of us into one big happy family. It includes crossdressers, transsexuals, gender-fluid, gender outlaws, androgynous, etc., etc., etc.
It's my understanding the term was originally coined because it sounded better than "transsexual." But technically, anyone who has identified with the opposite sex, whether they've come out to anyone or not, whether they are able to transition or not, whether they are able to under-go gender-confirming surgery or not, is transsexual. And to the best of my knowledge, there is nothing that says if a person chooses not to transition, that that makes them not transsexual.
I suppose however, that for the time it's fine to tell her you are "just transgender." But I can't help but think this is sorta like some guys who tell people they are lesbians, and then when they finally do tell someone they are really trans (FTM), it's like people totally Sybil-out on them. Just sorta seems like it's worse coming out "the second time around."
-Michael