so I actually have the broken GPU (the ATI one) I pulled from my iMac when I wrote this guide. I looked at the edge connector and curiously it doesn’t look like either one of your pictures. My card has a single notch in the center of the connector and both ends go all the way to the edge of the card. If you look closely at the first of the last 3 photos in the guide you can see it.

The wrong card shouldn’t damage the Mac. It just won’t work.

Maybe because it would interfere with the bracket you have to put back on in step 21?

Having done this procedure 6+ times now, I can tell you it's much easier if, instead of holding the panel up by the "bottom" as illustrated in the guide, instead raise it by the "left" side. It gives you (me) much more room to get fingers behind the connector.

May have been obvious to some, but this little adjustment didn't jump out at me until the 2nd or 3rd rebuild.

If you're going to try and substitute parts, I would start with the website I bought my video card from (http://www.impactcomputers.com/apple-ima...) and see if the video card(s) for that particular 2011 iMac match any of the part numbers aaaidan shows above.

I picked out one of the 2011 iMacs at random on the website and while the actual card looks the same (size and MXM connector) the heat sink is completely different. But you could try and dismantle the heat sink from the old and new cards and swap them. I would love to know if that works. An AMD Radeon HD6750M upgrade would be sweet :)

When I bought my iMac new, I remember the ATI card being the top-of-the-line option.