Depends on your mb bios and how it reads the spd profile of ram that on your mb. most mb will read the spd and set the ram to a stock speed 1066/1333. as your ram stock speed was 1066 it was set there. if you used 1600 speed ram it would have been 1333. to get to 666 speed go into your bios and look for xmp profile. it should be under over clock or dram speed setting.