Can't tell. You'll just have to try it. There's no guarantees when mixing ram. The mix will run at 667 if it's stable. You won't notice the difference in speed; I've tried both speeds independently, and didn't notice any difference in everyday tasks. Keep the ram in pairs; don't use 3 sticks. I've also used 6 gb (2x1 + 2x2 gb) and couldn't tell the difference over 4 gb. Best advice is sell the old ram first, then find used 2x2 gb of pc6400 on criagslist or anandtech for sale forum.