J.M.
Please watch this before you get a booster seat.
http://www.youtube.com/watch?v=azgBhZfcqaQ
I would stongly suggest against it. My seven year old still uses a 5 pt harness. My husband has talked to several poilce officers and they all say keep them in a 5 pt harness for as long as possible. Maybe we are over protective but at least she is safe. We have the Summit High Back at it can be a booster seat too. Again, it is a little pricy but you only have to get one seat instead of two for later. Good Luck, J.