You would want to be running dual channel on older computers, but it's not that big of an issue on newer systems running a single DDR4 2666-2933 module. Older and slower ram in single channel mode affects fps much more than newer DDR4 ram. It mostly depends on the game you are playing and how much it's affected by slower ram speeds and single vs dual channel mode.
You might find you have no real performance drop, but I suggest getting a 2x4GB or 2x8GB kit to begin with or try to get a second 8GB module with the same specs as soon as possible if you experience a drop in fps. I personally wouldn't invest in 2x8GB for such an old system unless I was running at least an i7 CPU.
Also, unless you are running an i3/5/7 3000 series CPU like an i5-3570 or i7-3770, you won't be able to run the new module at it's advertised 1600MT/s, because of CPU and motherboard chipset limitations.