Apple Support, writing about the old Cheese Grater Mac Pros:
Mac Pro computers use PCIe (PCI Express) graphics cards. Learn which Apple-provided cards work with your Mac Pro.
This article applies only to video cards that originally shipped with a specified Mac Pro or were offered as an upgrade kit by Apple. Similar cards that were not provided by Apple may have compatibility issues and you should work with the vendor of that card to confirm compatibility.
Getting this one posted before (hopefully) we see a new Mac Pro…