I agree with JDS63 that it should not matter that you don't have a UEFI motherboard.
The procedure I follow when I change graphics cards is:
Uninstall the current graphics drivers.
Power down the PC. (Do
not restart.)
Swap in the new graphics card.
Power up. If all goes well, you'll boot into Windows, perhaps with the Microsoft basic graphics driver.
Install the latest drivers for the new card.
Sometimes, I have needed to clear the CMOS to get the motherboard to boot with the new card. On your motherboard, that's done with a jumper. See your manual. (It's available for download as a PDF.)
Some of what you write doesn't seem to exactly match the motherboard in your specs.
For example, the Ga-p55-usb3 has only 2 slots that would accept a PCI-E X16 graphics card. One of those is X4 electrically, so you'd want to use the one nearest the CPU. You couldn't put a PCI Express graphics card into a PCI slot. I'm also not sure what "light" you refer to. The only onboard LED I see is one in the corner of the board that indicates CPU activity. (Sorta cool, but not all that useful.)
Best of luck.