Good luck! It's really good that you are going into with preferences but take it as it comes, not many do that but you truly give yourself a chance to really see which sorority is best for you to join.
I think though when you go to grad school, they are not any graduate chapters but just alumnae chapters based on geographical locations and cities. Although, some please correct me on that if I am wrong but that was the gist I heard.
Just keep in mind though that sororities are more than partying, there is more to it than that. I rushed when I was a junior and it was a good time even though things never worked out, but you honestly have nothing to lose but it's very difficult to pull off (i.e. get a bid). Just have a good time while you are there, get to know the girls in different sororities and just have fun (I really cannot finish my thoughts today lol)
It's better that it's before school b/c if the worst happens, not everyone will be in our business about it the next day after rush ends so there is that advantage. Also, you do not have to worry about school work so really it's a blessing in disguise, but I can understand how that would be difficult.
Keep us updated, I would love to hear how things turn out!