Which is your motherboard. If you have 4 dimm slots in your motherboard, you'll be able to add another 2 sticks of 8GB, if your motherboard support more than 16GB. (so you'll end having 2x4GB + 2x8GB = 24GB) It can have some compatibility issues, but if you can, try to find same model as you have with 8GB. I've never had any of this issue, but always looked for same ram speed and also latencies.