For theater use, we recommend a 5.1 or 7.1 speaker set. The speaker set you choose depends on the length of your room. If your room is less than 20 feet deep, you can use 5.1. If your room is longer than 20 feet deep, then you can use 5.1 also, but you may benefit from the 7.1 set up.
The reason 7.1 came around was to add two mid-side speakers in between the front and the back of the room to sort of tie in the sound in the gap from front to back. These speakers mount right next to your seating.