Mac Pro 1,1 GPU upgrade

[SuperSwordGaming]

So I just got a Mac Pro 1,1 in a trade for some stuff I don't need or want. I'm going to be upgrading my CPUs with a pair of x5355's (or maybe 5365's if I'm convinced its worth it) and I noticed the GPU is lackluster. Can anyone recommend a GPU upgrade for it? Something that will work good enough for some editing if need be. And is it possible to force update from El Capitan to the newest OS? Thanks in advance!
(Sorry, first time Mac user, with the exception of some PPC PowerMac G5's)

 

[hwojtek]

There's a zillion threads about it. Anything between 4870 and 7970 in short. Search this forum for details and refer to netkas.org if need be.

 

[SuperSwordGaming]

There's a zillion threads about it. Anything between 4870 and 7970 in short. Search this forum for details and refer to netkas.org if need be.

Does it need to be a particular brand, do the bios need to be flashed? I'm sorry, like I said, I'm new to all this.

 

[jbarley]

El Capitan (with the aide of some hacks) is as far as a Mac Pro 1,1 can go.
As far as Apple is concerned, Lion was the end of the line for the 1,1/2,1 MacPros.

Some good info here...
https://forums.macrumors.com/threads/2006-2007-mac-pro-1-1-2-1-and-os-x-el-capitan.1890435/