From my understanding, when the memory is mismatched the higher speed memory is downgraded to the lower memory's speed. I also know that even though they will work, mixing memory is not recommended. Check in that forum on that site or this one for memory overclocking. I was happy to finally get mine to 1600MHz. I got 12G from Fry's on sale for $119 per triple channel pack (3x2G sticks per pack) item #5982474 Patriot G series. I've tried 3 different coolers with that board (V8/V10, some other Fry's junk and Thermalright 120 xtreme. The xtreme was by far the best but I have been checking into the watercooling aspect of it. Check in the memory section of your bios for the current speed and timing.