"Your GPU does not support UEFI" on a UEFI system.

I have a couple of questions that have been confusing me for a little bit and I would like to see if someone wouldn't mind helping me.

I have a Alienware X51 2020BK. It came with Windows 7 Home Premium 64-bit, and I install Windows 8.1 Pro UEFI on the computer. The computer, in all, runs perfectly fine and has no problems.

I have an NVIDIA Geforce GT 545, and my BIOS is A13, if that's necessary.

1. How do I tell if my GPU is OEM or not, and if it supports UEFI/SecureBoot? I don't understand what the difference is and don't know which card I have, which may or may not matter for my issue. Are either of these UEFI-supported?
2. My computer runs fine on UEFI in Windows 8, but I can't boot to legacy devices. Everytime I try to run a legacy CD, it gives me the big "Please Insert Proper Boot Device" error on a black screen. Is there a reason I can't boot to Legacy on a machine that came pre-installed with an MBR/Non-UEFI (Whatever that's referred to as)?
3. Once I'm in the Boot Manager, it gives me the option to change boot type i.e. Legacy or UEFI. When I select UEFI if I have previously selected Legacy (to change it back to normal settings), It tells me that my GPU is does not support UEFI, which I'm (I believe) running Windows 8.1 through UEFI, so obviously it should support it.

Thanks in the future, if I need to redirect a question to another area, please let me know. You guys are the best.