Your MacPro can support quite a few graphics boards. But!! The primary needs to be the one Apple sold with the MacPro systems. Its not altered (unlike the iMac (MXM bds) its just Apple needed a way to have the system access a known graphics board as the Xenon CPU’s don’t offer an internal graphics service unlike the Laptop and other desktop systems Apple makes.
Think of it this way… Every 2010 15” MacBook Pro and 27” iMac onward has dual graphics! The Intel graphics are the default graphics in these systems. And the needed driver support is within the EFI firmware. Within the early MacPro systems the default graphics is the video card sold with the system.