I don't think there is anything wrong with saying your boyfriend (your serious boyfriend, not your "whatever" from the night before) is in so and so fraternity. No sorority WANTS to have bad relations with a fraternity, and if you are on good terms with one of them, it could work in your favor that if you became a member you could maybe smooth the waters a bit.
But on the other hand don't come in wearing your boyfriend's letters and think you are the cat's a$$ because of it. No one likes attitude and it's bad enough to have it on your own, let alone having "achieved" it through a man.
Something else that may be helpful is to keep a rush journal (well hidden, of course) to vent all your feelings/thoughts at the end of the day. That way if the first idea you have of XYZ is "what geeks!" you won't feel like such an idiot when you're overjoyed that they are your sisters at the end of rush week. You can just rip out that page and shred it.
