I have Gskill 3433 CAS 16 4x16GB on my x3900 (Gigabyte Master x570), boots stable at 3133MHZ (CAS16), will not boot at all at 3200 (CAS16,17 or 18), also does not work at anything less than CAS16 all the way down to 2666, so I'd keep that in mind.
I have a second set of Gskill 64GB 3200 (CAS 16) these are much cheaper and do about the same 3000MHZ (CAS16 slightly tighter timings).
Note: Stable is Prime95 (23 threads) using 62GB of memory for at least 6 hours. Simply put if it boots it seems stable on this MB, but booting is a royal pain it you mess up can take 30 minutes to straighten it self out and much swearing (not sure if that helps, but I feel better doing it).
I'm often surprised the number of people with 16GB on a 12 core or looking at a 16 core chip. I kind of figure you need 4GB per core or 2GB per thread so 48GB would be a sweet spot and 32GB the minimum I'd want or either 12 or 16 cores.