An experience points system is easy, so is a member system. I can give you a general idea on how to do it right here, or if you dont know anything about programming, AIM me at BurgerDog50 and I will happily code something for you. Anyways, to make an experience points system, what I would do is this:
In mysql, have a column for last deposit, and a column for total experience points.
When the user deposits, record the date such as 4/14/06 to the last deposit column.
To stop users from depositing more than once a day, check the current date against the date in the last deposit column. If they are equal, stop them from depositing.
To record experience points, simply take the current amount of experience points of the user, add any experience points they had gotten through the day to the current amount of experience points, then update the total experience points column with the new amount.
Again, if you cant code this by yourself, AIM me at BurgerDog50 and I will code it for you.