@ChaosLord, How can you be absolutely sure there's no 1333 MHz capable RAM ..? 
(but signal integrity will be a pain)
DDR = Double Data Rate. So a 1333MHz stick is running at half.. 666.5Mhz. However as it's double data rate RAM it can transfer twice as much data on one I/O bus clock, than the actual IO frequency so in fact it works out at 1333MHz.
So to if I break it down. 1333MHz is the data transfer rate. The I/O clock is 666.5MHz.
However 1333MHz is outdated by today's standards and the fastest DDR3 RAM is DDR3-3000.
3000MHz is acheived by overclocking from the factory. However MHz isn't the entire picture as you have SPD Latency which if far more important.
