Boosters are stackable so they will just keep adding to each other giving you more data or minutes.
So grab a bunch while they're on sale and get enough data and minutes to last you the whole year if you want.
I've stocked up on boosters, Harry, and they will just be added one of top of the other. It's an awesome deal the double boosters and I bought 3 x $25 1000 minutes = 3000 minutes and also 3 x $30 2GB = 6GB in total :D
That on top of the boosters I still hadn't left :)