How to choose RAM for the processor?

Question

RAM

Hello! I do not rummage in iron, so I ask for help.

1. What RAM to choose for this assembly?
Need DDR4 8GB (2pcs) for AMD Ryzen 5 2600X
Somewhere I read that this processor needs two-ranked brackets, is this mandatory?
Tell me which oz is better to choose?

1) ASRock B450M-HDV R4.0 AM4 motherboard
2) AMD Ryzen 5 2600X BOX processor
3) GIGABYTE GeForce GTX 1660 SUPER OC [GV-N166SOC-6GD 1.0] video card
4) RAM ???
5) be quiet! SYSTEM POWER 9 500W [BN246]
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6) Enclosure GiNZZU C200 [17142] Micro-ATX, Standard-ATX Mid-Tower
7) SSD Samsung 860 EVO MZ-76E500BW 500 GB
8) 23.8" Monitor Asus VA24EHE

PS Another question about the monitor: looking for 24' | HD | IPC | 75 Hz | And with NVIDIA G-Sync Compatible technology
I found only one option https://www.asus.com/Monitors/VA24EHE
There are no reviews on it, so I'm interested in the opinion of those who can say something about the characteristics.