If you have a multiple of 3 sticks of RAM it'll operate in tri-channel mode. Adding 2 to the 4, will make 6 (try to keep up).

There's two different ways of indicating RAM speed. One is to identify the way the clock is read (DDR/2/3), and the clock speed 1333 MHz. The other is the "PC" speed, which combines the different data-rates with the clock to give an over-all view of how different types of RAM compare to each other. DDR3 1333 is the same as PC-10600.

Pioneer PDP-5020FD, Marantz SR6011
Axiom M5HP, VP160HP, QS8
Sony PS4, surround backs