What does onboard RAM have to do with DirectX. Assuming DX12 means DirectX 12. Shouldn't you be more concerned with what Video card you are going to use? A quick Google shows that board as only having 1 PCIe x16 slot, so no SLI or Crossfire for you I guess. Nice board by the way but isn't that a bit of overkill for a desktop gaming PC. I assume gaming as you mentioned DX12, or are you into video editing or something similar?