Answer to: Black screen when try to installing a new OSX!I think that the problem is in the GPU. You need to have the stock GPU, because the other ones are Non-EFI GPU’s. I don’t know if the Clover EFI bootloader will help, because I never tried it.
Answer to: 2009 updating to High SierraHello Maciek, I have the same Mac Pro, only with a GTX 1060. From what i saw, there are macOS Cuda drivers for macOS 10.13.x ( http://www.nvidia.com/object/macosx-cuda-387.128-driver.html ) and the drivers for the Quadro and Geforce GPU's ( http://www.nvidia.com/download/driverRes...-us ). So practically, yes you can install the new macOS High Sierra. The only problem is that when you upgrade it, I don't think that the GPU is going to work until you install the drivers, so it's "safer" for you to do it with a stock GPU. It might work, but I'm not totally sure, because I never tried to do it.