It's best just to get 1 set of DRAM in the amount you want, mixing sets, even of the same exact model can often be problematic,,,,see this problem daily here at Toms and other forums, mixing back with DDR and DDR2 wasn't so much problematic as there were basically just set parameters, but todays DDR3 is all over the place with timings voltages, etc, different ICs in use and DDR3 is made to much higher tolerances