That goes to what I said earlier, the DRAM manufacturers offer such a variety to fulfill the needs, the sticks in a given set have been tested that they all play together (also why quite often a 4 stick set is a little more expensive than buying two 2 stick sets, it takes more testing to find 4 that will all play nice, it's possible you could pull 10 sticks right off the assembly line and maybe only be able to get 4 sticks that all played nice at spec. If it were as easy to just mix DRAM as many seem to think it is, the manufacturers would simply sell 1 stick packages and let everybody buy as many as they want and make bigger profits, they don't because they would go broke between support and RMA costs