It's not a good idea to mix DRAM, it can be problematic, even sticks of the same exact model...it's always best to get a single package of the amount of DRAM you want - that way you get sticks that have been tested to work together, and are programmed (for XMP) for that set and number of sticks, as advanced timings will differ from a single stick to a two stick set to a four stick set.... This is primarily why say a 4 stick set costs more than 2 two stick sets, it's harder to put together 4 sticks that play nice than it is to find two that will play together.....Tolerances in DRAM are so tight and exacting (especially in higher freqs