Here's something to consider....
If you go with a replacement kit.....in general the odds of it working are better than if you add a stick.
Also....16GB seems to be the place to be these days. 8GB is getting to be not enough.
It's your call.
I'm guessing you have a 4GB stick in the machine now.
If you want to go to 8GB shop for an exact match for the stick you have.
There is no guarantee the 2 sticks will play nice together.
Better odds are going for a 2x4GB kit from the mobo maker QVL or visit crucial. Crucial