Obviously you are up to speed with the fact that everything should match. That is good.
The old RAM isn't listed for your current board, as you mentioned, but that could just mean that nobody took the time to test it in that specific board.
Obviously it is working now, so it must be compatible. To that end adding more of the same should be perfectly fine.
All I can see that would cause the motherboard to reject more of the same would be if there was some cap on how much RAM at a given speed it can handle.
If you can look it up online that anybody got some higher figure like 8 GBs of 2000 RAM to work on the same board, then its probably fine if you just get more of the same.
In all likelihood there will be no problems if you get more of the same. The likelihood of the above being a problem is pretty slim.
The likelihood of receiving DOA sticks is higher than the likelihood of configuration problems I think.
Thanks for the reply Raiddinn,
Quick question, are you saying that I should try and get RAM that has a speed of DDR3 2000?