Knowing my situation and needs, do you have a better suggestion?
I keep a Sapphire Dual-X HD 7970 3 gb which I EFI flashed as a spare GPU.
I personally would not use any NVIDIa cards in Mojave as there is no official NVIDIa support since Sierra / High Sierra
These are the AMD GPUs that can be EFI flashed and will then show the EFI bootscreen in Mojave.
They all have ' Metal Support '.
R9 270 X ( possibly but I suspect "yes" )
HD 7950 ( yes, nice card but HD 7970 is faster, better )
HD 7970 ( yes, very nice card )
R9 280 ( yes )
R9 280X ( yes, very nice card )
I have flashed all of these cards to show bootscreens - I started from scratch and learned slowly but steadily with a lot of help from members on the Netkas site shown below.
The above cards can be EFI flashed; the technique is freely available on the Netkas site.
It is not especially difficult to EFI flash an AMD PC GPU just tedious and I advise reading EVERY post in relevant threads
For HD 7xxxx cards
http://forum.netkas.org/index.php/topic,5619.0.html
My Sapphire Dual-X 3gb HD 7970 works perfectly with EFI bootscreens in Mojave 10.14.6 ( build 18G103 )
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NOTE : The GPUs listed above ALL have "dual BIOS : switches which simply means that they have TWO BIOSs.
HOW TO EFI flash
Before Mac Pro startup set the GPU to BIOS Switch Position #2.
Look in your High Sierra / Mojave System info for your GPU's "DEVICE I.D. write is down
Then start up Windows 7 ( preferrably ) then you "DUMP" ( download the ROM from the card- using GPUz then SAVE it and a copy somewhere.
Then you shutdown - remove your Win7 HDD - then start up in Mac OS and run the Netkas script which will add the EFI boot info to the ROM you just DUMPED.
Save THIS ROM with a name eg: 7970_EFI_rom
Shut down, re-insert your Win 7HDD - boot into Win 7.
Now download and start the latest version of AtiWInFlash.
Select your GPU from the list above and FLASH your new eg : 7970_EFI_ROM to your eg: HD 7970.
Done !
Bear in mind that if you decide to buy and EFI flash any of the HD 7xxx GPUs , , ,you have a lot of reading and note taking to do in the relevant Netkas forums first.
I am willing to help you should you decide to go ahead with flashing a HD 7xxx card.
I highly recommend buying either an MSI R9 280X or a Sapphire Dual-X 7970.