After voting to close a question, users who voted should be notified whether or not the question was closed.
I understand that the criteria for determining whether or not the question was closed is a bit tricky. It would probably have to be based on whether the question was closed N number of hours after the first close vote was cast.
This notification will help users vote more intelligently when casting close votes by allowing them to see when they voted properly and when they didn't.
Currently, once a close vote is cast, unless you keep the question open in another tab or mark it as a favorite, it's easy to lose track of it.