Both of those ram sticks are located into Gigabyte's GA-H61MA-D2V Memory Support list (http://download.gigabyte.us/FileList/Memory/mb_memory_ga-h61ma-d2v_v2.x.pdf), so both of them should be fine... and it is not 1666MHz, the F3-10666CL9D-8GBRL model runs at 1333MHz while the F3-12800CL8D-8GBECO model runs at 1600MHz (and in order to work will be downgraded to 1333MHz according to Gigabyte memory chart)