It's pretty much like this.
Provided you get the exact same stick of 8GB as you currently have, there are more chance than not that it'll work.
If you get a new pair of 2 X 8GB sticks, it'll be garuanteed to work with each other.
So if you have the funds for two 8GB sticks, go for it, there's no garuantee that the new 8GB, even if the exact same model, will run with the old one.