Stick to 3600 unless you want to spend a lot of time fiddling. Somewhere between 3800 and 4000+, most Zen 3 CPU samples will drop the internal fabric clock in half (because it can't run higher) and that actually hurts performance. The gain by going over 3600 is minimal in most cases. Feel free to buy 3800 MT/s memory if you don't mind spending the extra for almost no gain.