Too bad you didn't ask what memory you should buy before you ordered.So I have 2x2Gb A-data DDR3-2000Mhz@1900Mhz 1.65V, and just ordered 1x4Gb Kingston HyperX Genesis DDR3-1866Mhz 1.65V.
I assume I will be able to run the memory together, but what will be the best way to fill the memory slots?
Too bad you didn't ask what memory you should buy before you ordered.
We would have told you to buy a 4GB kit rather than a single 4GB DIMM.
To answer you configuration question... Install the two A-Data DIMMs in slots 2 and 4, the Kingston module in slot 1.
But I cannot understand -- given memory prices -- why you would want to make a dual-channel configuration into a single-channel setup by only adding a single stick of RAM. Doesn't really make sense . . .