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 built-in default OS X nVidia driver supports older-generation graphics cards like the GTX 680 (you still won't have a bootscreen, but the OS installer and the OS itself will never be a black screen, and you won't have to manually install any drivers).
However, the built-in driver doesn't support the...
How to install a PC nVidia card in a Mac Pro and how to safely do OS updates
The built-in default OS X nVidia driver supports older-generation graphics cards like the GTX 680 (you still won't have a bootscreen, but the OS installer and the OS itself will never be a black screen, and you won't have to manually install any drivers).
However, the built-in driver doesn't support the...
How to install a PC nVidia card in a Mac Pro and how to safely do OS updates