Make and model of your PSU as well as it's age? You should be fine to drop the GPU in the PCI-E slot but I would be concerned if the lack of an UEFI BIOS environment. All the GPU's coming from the factory(even replacement units) are based on UEFI BIOS so dropping in a GPU with UEFI BIOS onto a motherboard that lacks an UEFI BIOS will result in a black screen, in spite of meeting power and slot requirements.
Can you list your full system's specs?