It'll run at the slower speed.

Yes, you can mix them. The motherboard will run all RAM at the speed of the slowest stick — 2666MHz in your case. So the new 3200MHz stick gets held back.

Whether you’ll notice depends on what you’re doing. For gaming or heavy multitasking, you’re leaving some performance on the table. For everyday web browsing and spreadsheets? You won’t feel it.

One more thing: make sure both sticks have the same voltage and similar timings (CAS latency, etc.). If they don’t, the system might get unstable and refuse to boot. But usually it just auto-negotiates to the slower profile.

If you already own the 3200MHz stick, this is fine. If you’re buying new, just get another 2666MHz stick and avoid the headache.