To me ... sounds like you building your first computer
and you have lack of knowledge about how to do it , what supports what Read your motherboard description on the manufacturer website :
PRIME H270-PLUS/CSM
If you doing that , you going to find out , the only mem supported by your motherboard is DDR4
From HERE you can downlad the PRIME_H270-PLUS_MemoryQVL list
If you are going to be using 3x Vega 64 for mining and planning to add more GPUs later. " make sure you have at least 4GB of system memory
but in your case 8GB would be much better choice Ya it's been close to 15yrs since I build my last PC so am a bit Rusty.
Thanks for the link and advice. I'll stick with 8GB for this build.