1. You are no exception to any rule here. Everyone that operates their own web page or message board has to pay a fee to the server. You knew this when you started the board. To tell people now that you need money to continue it really isn't fair to the people who have patronized your board and made it successful.
2. You mentioned that you had too many posts to handle. That would indicate a successful operation, which is not a negative thing. If you don't have the time to maintain this board, then maybe you should ask for assistance from someone. I don't see how getting $20.00 from everyone on here would give you extra time.
3. I'm sorry, I still can't understand how you can justify asking for the money, then on top of that, telling these people that you won't guarantee what it will be used for. If you get the money, you should at least put it toward increasing the memory on your computer or something that would assist you in the board maintenance. Asking for it as a gift is understandable to avoid the tax problems, but these people, especially the ones who have already sent part of their personal earnings, are at least owed a guarantee as to its use.