I'm not certain that this is the correct Stack Exchange forum for this question... let me know.
I must acknowledge this iFixit Q&A "Can the graphics card of the 24" iMac be upgraded?". I have read through the question and most of the answers and comments.
My model is the entry level early 2008 24" iMac 2.8GHz processor, 2 GB RAM, 320 GB HD, SuperDrive, etc.
My particular model shipped with the ATI Radeon HD 2600 PRO with 256MB of VRAM.
Many years ago I upgraded to 6GB RAM and installed a 2TB Seagate HDD with an SSD flash module (to mimic a “Fusion“ drive). These upgrades still work perfectly.
Recently the ATI Radeon video card expired… at least I used the four LEDs at the bottom edge of the machine to diagnose this as the issue. (Refer to this iFixit Q&A "iMac Intel 24" EMC 2134 and 2211 Diagnostic LED's".)
I successfully procured a replacement - the NVIDIA GeForce 8800 GS with 512 MB of VRAM - a BTO upgrade option for the original machine.
I assumed that this card would be a part of another successful “upgrade” project.
Despite A LOT of advice to the contrary, my particular iMac has a video card slot that physically accepts both types of cards (the ATI or the NVIDIA).
Whether the replacement card functions as expected in that slot is something I'll have to provide advice on later...
So while the replacement NVIDIA GeForce 8800 GS board physically fits the space within the chassis and it physically fits into the video card slot, it DOES NOT fit the heat sink.
The NVIDIA GeForce 8800 GS board has 8 RAM chips in different locations to the ATI board's 4 RAM chips. The heat sink shipped with my iMac (for the ATI Radeon video card) also has a different size clamping bracket (the bracket that clamps the heat sink plate to the video card) and also has a different layout for the screws.
The photo below details the two video cards. The video card on the left is NVIDIA GeForce 8800 GS and the video card on the right is the ATI Radeon HD 2600 PRO, with the heatsink above (mirrored).
The photo below details NVIDIA GeForce video card when compared to the clamping bracket from the ATI Radeon video card.
I am now facing a decision…
- Do nothing, give up on the project and place my time and energy into something more productive.
- Sell the NVIDIA GeForce 8800 GS and buy an ATI Radeon HD 2600 PRO.
- Keep the NVIDIA GeForce 8800 GS and buy a heatsink to match that card.
I acknowledge that there may be other issues with this proposed conversion that I don’t understand or possess the knowledge and skill to resolve, such as upgrades to the video card BIOS and EFi. Advice on how to overcome or avoid these issues in particular would be appreciated.