Should I buy the exact same RAM stick or can I mix different sticks?

Buy the exact same brand, model, speed, and timings. Mixing RAM is a gamble—different die batches or PCB revisions can cause instability even if specs match on paper. If you can’t find the same stick, get a matched pair (2x16GB) and sell your old one. It’s a few bucks more but saves you troubleshooting headaches.