I would NOT go with 1866 on vishera. Yes it's "supported" but i have seen many nightmare like scenarios where people couldn't get it to run 1866 no matter what they did, it got to the point where tiny tom logan said screw it and just ran his RAM at 1600. I'm running 16gigs @1700mhz (small oc from 1600mhz) on stock timings (9-9-9-24), i really didn't notice any difference at all. So personally i would go with 16 gigs at a lower speed than 8 gigs at one tier faster speed. You also need to get into timings and all of that technical garb. To make a long story short, just stick with the 16 gigs @1600mhz, you save yourself the potential headache.