1333 MHz memory should work OK with the X4 860K with two modules used in a motherboard with two DIMM slots. If motherboard has four DIMM slots with DIMM's in two slots, memory frequency will default to 1066 MHz. If all four slots populated, then memory frequency will default to 800 MHz.
Performance loss from 1866 MHz down to 1333 MHz memory will be around 10%, depending on application/game.