You and I are in the same boat. Same late 2012 iMac. Same GPU. I've done some research and I found that the only way to upgrade the GPU is to add an external thunderbolt chassis and install an nVidia card in that. Biggest downside besides the expense is the fact that it will not pass video back to your iMac. You'll have to add an external monitor. Right now, it's about a grand in outlay. So, for me, I'm to have to get to the point where I'm mad at my Mac on a regular basis to do this.