1866 is the fastest it will support but depending on the motherboard and quality of the chip you maybe able to use 2133 memory. Couple of things to keep in mind though is that the cpu doesn't really gain anything from using faster memory directly however if you overclock the system through the bus instead of the multiplier you will see a better overall performance gain. Now with that being said running the memory at that speed be it by xmp or overclocking from 1866-2133 will require additional voltage from the CPU-NB which has the unfortunate risk of damaging the memory controller.