my new cpu and 2x RX480 8 Go are installed now and i checked about the GTX570 that runs in DX12 when single [ DX11 when 2 or more ]
first results are very surprising …
starting with the available ram for the amd : only 4095 Mo while the card itself has 8 Go with DXDiagx64 , with DXDiagx86 the card has 24 Go
images are [2 x 1440] x 900 
https://picload.org/image/roraocrc/nvidia_w10_000.jpg
it is the same with the nvidia : only 4095 Mo with DXDiagx64 , with DXDiagx86 the card has 17 Go . [ so that seems the aperture is half the system ram : 16 Go ; shared by the 3 cards…]
there is another bug : in the nvidia control panel , the D3D Api Version is reported as 11.2 while the two DXDiags shows it as 11.4
https://picload.org/image/roraocrp/nvidia_w10_00.jpg
https://picload.org/image/roraocdg/nvidia_w10_0.jpg
last but not least : the HDCP Chain that the nvidia panel reports has broken…i think that comes from the soundcard that needs a video hdmi input to add the sound to the video out and export it to the receiver and then the monitor . the Gtx being HDMI 1.4a while the rest is only 1.3 …
so that looks promising … soon we will play in x64…that also “explain” why DX12 needs a new os [ that fails…] like DX10 that needed Vista … [ that failed …]