Instead of a speech, if I were in the audience I would like to know what you remembered personally about the great or grandparents if you had that knowlege. Invite those who would know to help you tell the family story. I woulld place family tree charts as handouts on the entrance to the the room where those who want to follow along could do so. Folklore with a little bit of history sneaked in is what is interesting. Make the family history -more human.