well....to solve your current situation..have you tried staying over @ your gf's place? You really can't tell your roommate to leave his g/f @ her place if you are in turn having yours...I'm assuming you have equal rights to the usage of the room and that is what comes in having a roommate, you gotta put up with other ppl's shit. It's all a matter of compromise. If you are serious about not having his girl in your room, you can't have yours...simple as that.
As to whether your current roommate and the situation for next year, just tell him that your new roommates found someone else and that you (as a new house) voted and didn't have space for him. Just tell your new roommates what an @$$ or his habits and if they are somewhat SANE, they will go along with the plan. I would. And to those who think this is backstabbing, go :fawk: off...i would LOVE to know what kind of person the new roommate would be and whether i would want that kind of person in the house that i am living in. If i'm putting up money for it....i'm not going to want to have to deal with it.
If you want her out of hte room...just tell them you are studying an dyou need it quiet or put on some headphones. Go to bed early and turn off the lights. Act like you turned anal......sometimes the best way to get anything done.
Hope this helps a little bit and don't worry aobut a month and a half...it's NOTHING compared to havng to deal with it for a YEAR...