If the 12GB of RAM was 3 sticks, likely yes, but the 12800 RAM would run at the slower speeds of the other RAM. You might have to set some voltages higher too, but don't know too much about that... think you might have to raise your QPI voltage if you put it all in. Also, with 18GB of RAM, you would need a Professional version of Windows as Home Premium only supports up to 16GB.