It's a 50/50 crapshoot, which is why none of the memory manufacturers will guarantee mixing packages of DRAM. The forums are full of threads of people trying this and failing. Sometimes they play other times they don't. We even commonly see people buying 2 sets of 2 sticks at the same time to save $3-4 dollars or whatever over the cost of a 4 sticks set. That 4 stick set cost more because it commonly takes more testing to find 4 sticks that will all play together then it does to find 2 sticks that will