Add another 8GB stick.

Yes. Just buy a matching 8GB stick at 2666MHz. Don’t replace both with faster RAM.

Adding a second stick gives you dual-channel memory, which is a real performance boost — often 10-20% in CPU-bound tasks. That’s way more than you’d get from faster RAM on a system that already runs 2666MHz. Going from 2666 to, say, 3200 might give you 1-3% in most real-world use. Not worth the money or the hassle of selling your old stick.

Faster RAM only matters if your CPU and motherboard actually benefit from it. Many mid-range or older systems top out at 2666 or 2933 anyway, so paying for 3200 is wasted. Check your motherboard specs, but odds are you’re fine where you are.

Just get the exact same model if you can. Mixing brands sometimes works, sometimes doesn’t. Spend the $25 and move on.