Mix RAM CMD4GX3M2A1600C8 with CMX4GX3M2A1600C8

I think you can mix them without having any problems. They are both running at the same speed 1600mhz and they have the same latencies, so you shouldn't get any performance decrease.

Why are we speaking english?

Ohh well I'm not from corsair as you might have understood. I think you should wait for someone from corsair to answer. Because I'm not entirely sure you can mix them. I think you can but I'm not 100 percent sure.

The replacement for your kit is CMP4GX3M2A1600C8, which is the same regular black Dominator, but with the parametric data connector to support Corsair Link.

As you're going from two DIMMs to four DIMMs, you may run into some problems as the memory controller will be addressing twice as many ranks. We test and qualify specific 4 and 6 modules kits so that the modules work together, and so it's possible that using two different dual-channel kits won't work so well together. This would be complicated by the fact the new kit may use different ICs to your initial kit, since it's newer.

That said, it won't necessarily not work either, and you'd really have to try it and see. It's certainly possible they will work perfectly without you needing to do anything. Just make sure the memory timings and voltage are set correctly. If you do hit problems, you can trying increasing the voltage to the memory controller slightly, or reduce the speed from 1600MHz to 1333MHz, as this puts let strain on the MCH and should allow them to work.

The alternative is to go for a dedicated 8GB or 16GB kit, using 4GB DIMMs.