iMac 7,1 - Graphics Card Replacement

[kafkaesk]

Hey folks!

I own an old iMac 2.8 24" 7,1 Mid 2007 (BTO/CTO) with the ATI Radeon HD 2600 Pro GPU.
This card recently started to fail, producing artefacts on the screen, until finally the card completely died.
I then bought a NVidia GeForce 8800 GS, plus - additionally - the fitting heatsink from another vendor on eBay.

Now, the iMac doesn't boot into macOS anymore, the progress bar stops at around 3/4, and the iMac restarts.
Of course, I already resetted NVRAM and SMC stuff.

I then tried like everything that came to my mind:
1) Fresh install of El Capitan -> doesn't work, stops as well at 3/4 of progress bar, whatever I tried USB or DVD
2) Windows 7 / 10 USB BootStick -> black screen, no difference between USB or DVD install
3) Ubuntu and other suitable Linux Distros -> Boot Menu appears, afterwards no screen...

Verbose modes work, but it seems as if the "problem" comes out when the iMac tries to switch into a graphic mode that is beyond VGA.

So I inserted the original mac OS DVD (Lion?) to run the Apple Diagnostics (see screenshots attached).
Because of the TGOP fault displayed, I tried two other sensors I had over, with no effort: same fault.

But since it also showed "unknown graphics card", I then tried the Mac_GPU_Flash.zip by @The_Croupier out of this post ...
When I try to check my card with nvflash --list ... it says that there is no compatible GPU installed.
Trying nvflash --lspci, the card shows up.

Honestly, I don't know what to do anymore at this point. For me, it looks like I bought a Windows-GPU taken out of a normal notebook.

Maybe someone of you can help me, I don't want to throw away the iMac which is still sufficient for my father to check mail and surf a bit the internet.

Greetings, Daniel.

 

[kafkaesk]

Okay... now I'm a bit confused whether I told you the truth since I found an old thread of mine here.
Due to the fact that the post is from 2017, I'm now relatively sure I bought a 9800M card back then, but never replaced it because my father was fine with some of those artefacts.
And since the graphics card now (6 years later) really died, I swapped it and it now doesn't work.

So, maybe, the better question of mine should be: How can I get the 9800M being recognised by the iMac, how can I flash it so that it works.
Funnily: The heatsinks of 8800GS and 9800M seem to be the same, because it fits perfectly.

 

[kafkaesk]

We have so many professionals and geeks here...
You seriously can't tell me that, after over 300 views, nobody has a clue how to help me.

 

[sassemblyman]

If your intent is to keep this machine running, which I respect and appreciate, I would advise you to purchase another 2600 pro and call it a day. The 8800s are also prone to failure. Pop a CPU upgrade in it for SSE4, an SSD, 6GB ram, and you can install whatever OS you'd like with acceptable performance patched in Monterey even. You can probably find a busted donor unit for less than you'll find the card.

If you want to put this same amount of effort into a machine that will actually perform very well and support modern OS with Metal GPU, I'd find a 21.5" 2011 iMac for next to zero dollars on a local listing and flash a Metal GPU which you can get for less than 30 bucks.