They would probably either lower your bandwidth to compensate for overuse...or charge you the appropriate amount depending on how far you've crossed the 2GB mark.
My isp is kind of strick,Thank god they have a limit of 60 gig's a month The first time you go over they warn you the 2nd time they suspend you for a while,Then the 3rd time you go over they cut you off and wont give you service ever again