First of all the motherboard has to support the sticks being used. Sounds like you are trying to use ddr2 - 667mhz and ddr3 - 1600mhz together, which you can't. They must be all the same type as either all ddr2 or all ddr3 depending on what your motherboard support. And if you use different speeds say 667 and 800 mhz together then the 800 mhz stick will run at 667 mhz speeds. Same thing goes for ddr3, all sticks will run at the slower speed if you don't use matching sticks.