Not sure if it would actually work. I’m in your same situation except mine works for a bit, then freezes up. I tried using a Radeon HD 4670 from a 2010 iMac, and my iMac didn’t show an image. Then again, I didn't let it boot all the way to see if it would actually show something. (Didn’t have enough time to properly test.) I need to get another GPU on my hands and see if the Mac will recognize it once in the fully booted stage. I also want to try and see if I can run it without the GPU and just use the integrated 9400M for the time being. That would get me along until I can get a Radeon HD 4870 to install.