Searching for an AMD GPU natively compatible with macOS for an old desktop with 250W PSU?

Hello everyone. I would like to get full graphics acceleration on my old desktop (Intel i3-530 8Gb of RAM). Considering that Clarkdale is not natively supported (and I don’t want to change macOS system files), I decided to buy an AMD GPU. I’m using Opencore (Legacy BIOS).

I will run maximum macOS High Sierra (so I don’t need a Metal compatible GPU). For now I’m running OS X Mountain Lion.

Since the PSU isn’t very capable, I could buy a Radeon HD 5000/6000 series gpu.

Thanks in advance.