I don't think 3970X is very choosy, though it depends on individual chip luck. Strix-E Gaming is good board, should have wide compatibility with latest bios.
Mid-range 3200/CL16 or 3600/CL18 from reputable brand will work. I am also fan of Crucial Ballistics, because they manufacture the dies as well (Crucial is sub-brand of Micron). With any other brand, it's lottery what is it that you get (It can be Samsung, Micron or Hynix, and any tier of quality).
As usual, while not necessary, buying a "kit" (all modules together) will increase a chance of all running well. It's easier for memory IO to have as much ranks in single module as possible. So 4x32GB is better than 8x16GB for example, even when both have 128GB total capacity.
In case you would want to run 256GB (8x32GB), those start to be a problem and it's the only time you should go for high-end memory to make sure it will be stable. Only few brands offer such as kits though, particularly G.Skill.
But since you already have "2 x 32 Gb HyperX 64GB KIT 3200MHz DDR4 CL16 Predator ", I would suggest buying another EXACT kit ( don't google the name, google the ID code such as "HX432C16PB3K2/64". Memory often gets different dies for the same name one year than other.