The problem with non-EFI (PC) graphics cards is that they aren't initialized by the Mac boot sequence, so the screen remains black until a compatible driver is loaded. The default OS X driver doesn't support the newest PC cards, so here's what you need to do:
Initial setup and preparation for installing the PC graphics card:
GUIDE: How to safely do OS updates when using PC nVidia cards in Mac Pro
Initial setup and preparation for installing the PC graphics card:
- Disable FileVault (whole disk encryption) if you have it. It does NOT work with non-EFI graphics cards and you won't be able to boot. Unfortunate...
GUIDE: How to safely do OS updates when using PC nVidia cards in Mac Pro