Now, I am not an expert in any way, shape or form, but I use two different RAM sticks (4+8), one from Intergral (4 GB) and one from Elixir (8 GB). The power consumption and the speeds are the same and I am not getting any problems that I would know of, but I did not test it like 'the big boys' and I don't know about the long term effect, but my PC is ON and hasn't exploded yet.
But it's known that for the best effect you should use two same sticks of RAM, but I don't think I've heard that the (if any) performance loss is a game changer.
(Not responsible if anything goes wrong after reading my answer.)
For the best performance either buy another stick that is exactly the same as the one you already have or buy a new set of ram. Two different ones might actually work from time to time, but they will not perform as well and will run into hiccups from time to time. If your ram is working efficiently together to it's max potential then it can potentially slow down other components that rely on it such as your CPU.
They might work just fine or they may be problematic and they might not work at all together...Even trying to mix the the same exact model of sticks can be and often is problematic...best bet is one package of the full amount of DRAM you want/need, then if you can get the old to play it's a bonus, else can sell the old off or put it in another computer