I am running 2x2gb and 1x4gb setup. 3 sticks total. Each was a different speed/timings. It took me forever but I finally got it figured out. You need to set the speed to the slowest timings of any of the sticks (higher numbers). You may need to lower the speed. I had to either set mine at 9-9-9-24 at 1333mhz or 11-11-11-29 at 1600mhz for it to be stable. I chose the first of the two.