somebody mentioned recently, that it works on PCI E 3 (what I didn’t know / questioned)
… with reduced speed, but I don’t find the post, which included a link to a technical advice
there it is GPU advice - #6 by Wolfgang and then further down 
Sorry, I don’t have enough knowledge on that as I normally use laptops and there I have no choice to upgrade singel components.
It doesn’t matter. The bandwidth used by DxO to process images with the gpu is minimal, the difference between pci-e 4 and pci-3 is negligible (even with a 1x pci-e 3 card!)
Thanks, Gonzalo and Wolfgang, for your responses!
I bought a used RTX 2070, and compared to the 750 the process time for a 25 MP image and DP-XD dropped with about a factor 8. Satisfied
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Good it worked!