Mac Pro 1,1 GPU Upgrade

[harveylong26]

Hey,
I'm planning on purchasing a MacPro 1,1 for 2 reasons

1. They're incredibly cheap to find on eBay in the UK.
2. You can bring them back up to spec using methods to run El Capitan.

The method I'll be using will require me to upgrade the GPU (well that's what the guy say in the video) and I was wondering what would be a good card to use in a 1,1 to run El Capitan.
I'm assuming it'll come with the stock GPU (eBay listing doesn't state)
I currently own an R7 250 1GB. Would I be able to use this or will I need to upgrade?

 

[cw48494]

If you can, try to pick up an HD 7950 or R9 280.

 

[harveylong26]

If you can, try to pick up an HD 7950 or R9 280.

Do these work out the box/plug and play with boot screens?

 

[cw48494]

Do these work out the box/plug and play with boot screens?

If you flash them or buy them pre flashed/mac variant.