3200MHz RAM vs 2666MHz RAM: Mixing speeds works but wastes performance

You can mix them, but the motherboard runs all RAM at the slowest stick’s speed — 2666MHz in your case — so the 3200MHz stick gets held back. For gaming or heavy multitasking, you’re leaving performance on the table; for everyday browsing, you won’t notice. If you’re buying new, just get another 2666MHz stick to avoid the headache.