Should I add another 8GB stick or buy a new 16GB kit for a RAM upgrade?

Add a single matching 8GB stick — it’s cheaper and usually works. A new 16GB kit costs $25-35, while a single 8GB DDR4 stick runs $12-18, saving you roughly half. As long as the speed and CL timing match, just plug it in. Modern boards handle minor mismatches by downclocking, and worst case it doesn’t POST and you return it. Only buy a full kit if building from scratch or overclocking.