basicly its due to the IMC on DIE limit INTEL only guarantee 6Dimms 12GB at D3-1066MHz 3CH for overclocking modules, rated above 1600Mhz, only for 3CH and 2GBX3 or 1GBX3 the battleneck of IMC limit of i7 now is 12GB 1600Mhz Max ( instable ) i tested GA-eX58 series, 1GBX6 no problem with D3-1600MHz, and if 2GBX3 max 1600Mhz bootable but instable.