those numbers are the speed of the memory 1866 being quicker than 1600, but where talking milliseconds lol. best thing is to take back cover off or sometimes its easy access and take a picture of the mem stick. then see on flee bay what's going. ?? yes you can put just 1 x 8gb in as opposed to 2 x 4 or leave in the 4 and just add the 8 also.