I would buy the exact same stick that you already have again to be 100% sure that it will work.
Well RAM is sold in kits for a reason. Some motherboards react very
sensitive to this and that is also the reason why the vendors do not
support RAM that is not bought in one kit.
Altough, I think the problem with newer intel chipsets is less. But I
would choose the safer path by buying RAM in one kit instead of loose
sticks. You can try to put in that 8GB HyperX but the ram should have
the same voltage, speed and cas numbers.
However some things changed. The 3 most critical things that should match are Timings, CAS latency and Voltage. If you have 2 ram of different speeds they will adjust to the ram at the lowest speed. I personally would't risk it and buy 1 of the exact same RAM that you already have.