My recamend is fill all slots dont see capacity. see mhz in board and ram.Reason mhz is the key to achive speed. if board support 1600mhz u insert 1333mhz its waste because not get full speed of ram slot. if board support 1333mhz u insert 1600mhz its also waste of money (1333mhz ram is cheaper than 1600mhz)also u not utilise full speed of ram. (2x4Gb=8Gb 1600mhz=3200mhz =3.2ghz) vs (2x8Gb=16Gb 1333mhz=2666mhz=2.6ghz).