I have a pc with the gigabyte ab350-gaming 3 motherboard and already installed is 2x4 3000mhz ddr4 ram (vegeance) and want to use up the 2 more slots. Could I install 2 x 16 2666hmz one?
Or should I stick with 2x8 3000mhz?
You "can" but it is most likely all other memory sticks will downgrade to the lowest speed stick you have installed. Also it is most likely that you will not be able to obtain dual\quad channel memory in that setup.
You are better just spending the money and either buying a memory stick kit or matching memory.