Always go quad kits not 2 x Dual to ensuire matched SPD tables.,
+4000
Just wasted three days trying to get 2 seperate dual channel but identical sets of 1866 16Gb (2x8gb) working on my system. They run fine at XMP settings until I try to get above 4.5ghz.
Just ordered a set of quad ones now after flogging my existing set. Its what a lot of serious overclockers recommend (including raja @ asus) so heed the warning!