Suggestion:
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.
Purpose:
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.