Get the SAME make and model for that new stick. I've found folk that do not want to dive into the specifications. That is, they call it the same spec but then you open off the FULL SPEC and find a new item in the spec that does not match the older sticks.
If you want to avoid all this work, get the same stick from the same maker and the same place you got the old sticks.
I have corsair ddr3 dual channel RAM 2X2 4GB. I use it with my Asus Dual channel motherboard without any issue. Now I have built a new computer the motherboard is Asus Rampage III it has triple channel. I use this dual channel RAM on this new built but I would like to make it triple channel. So is it possible this dual channel DDR3 RAM to use as triple channel by adding one 2GBS DDR3 RAM with the same specification.