There is no problem with 1600MHz rams. They will work on your motherboard but I strongly advice you not to add extra memory just by adding two extra cards and not removing the old ones.
If you really need more memory get 2x4gB sticks. Ram cards should be bought at the same time.
If you use any new cards with your current Kingston memory you will face crashes as most probably the new cards will have different specs. If you can get the exact same set of memory you currently have in your system then you are good to go.