Installing macOS El Capitan 10.11 would require a patched kernel, as the AMD Kernel Patches recommended on this site don't support any version of macOS older than High Sierra 10.13.x.
You should be able to run High Sierra with your legacy ATI HD4870 dGPU.
I am not sure if your GA-970A-D53P motherboard uses UEFI or an older legacy bios. If the later, then you will most likely require a Legacy installation of OpenCore, if you opt to use the most up to date boot loader along with the Vanilla installation of macOS for your old FX system.
There is a guide for an FX 8350 and Asus Sabertooth 990FX system in the success stories forum. It would probably be quite close to the OpenCore setup you would require with your similar FX system.
I have been playing around with my old AMD FX 8350 system (again) and decided to see if I could get it running macOS Big Sur. I used OpenCore 0.7.6 and followed the Dortania AMD Bulldozer guide when creating the EFI folder. I created my macOS Big Sur USB installer using the following...
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