50th Wedding Anniversary

I am planning my parents 50th. It will be in June04. I have already reserved a room at a restaurant and have the invitations almost ready to send. I am on the fence of whether to tell my parents, but not give details. There is alot of relatives that lives out of state. Also making sure I invite everyone is also an issue. My mother has already inquired, not wanting me to have a big party because of cost. Does it have to be a surprise? Also would like ideas on centerpieces. About 50 people invited.