How much performance will I gain by upgrading DDR4-2666 to DDR4-3200?

You may see slightly better 1% lows and smoother frame pacing, while average or maximum FPS only increases a little. Since the 5600 officially supports DDR4-3200 and you play at 1080p, upgrading can make sense if the faster kit is inexpensive, but it isn’t likely to feel like a major upgrade.

If you’re buying new memory, a 2x16GB DDR4-3600 CL16 kit would generally be a better target than just replacing 2x8GB DDR4-2666 with 2x8GB DDR4-3200. Otherwise, keep the current kit and save the money for a larger CPU upgrade, such as an X3D model, where supported.

For AAA games with graphics settings turned up, the GPU will often be the limiting factor, so the RAM change could make almost no noticeable difference. I’d prioritize the upgrade if you also need more capacity; moving from 16GB to 32GB may help more than simply increasing memory speed.

The improvement will probably be modest—around 3–5% in many situations, and potentially less depending on the timings of both memory kits. At 1080p, the Ryzen 5 5600 can benefit from faster memory, but the difference is usually more noticeable in benchmarks than during normal gameplay.