You can populate all 4 of your slots with 4GB of DDR3 1600MHz ram each giving you a total of 16GB 's of maximum ram upgrade. I would advise against mixing and matching sticks of ram while going for one stick of ram will only mean you will need to get rid of your existing 2x2GB kit on your system.
When you install ram in any two slots on your motherboard
menaing slots 1 and 2, you achieve dual channel memory mode. If you have all four slots populated by a kit(e.g 4x4GB) then both banks of ram are in dual channel.