Upgrade 24" iMac video card - ATI Radeon 256MB to NVIDIA 512MB

Before you read on I must acknowledge this iFixit Q&A Can the graphics card of the 24" iMac be upgraded?. I have read through this question and most of the answers and comments.

Background

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 this video card expired… at least I used the four LEDs at the bottom edge of the machine to diagnose this as the issue.

I successfully procured a replacement - the NVIDIA GeForce 8800 GS with 512 MB of VRAM, thinking that this card would also become another successful “upgrade” project.

Despite A LOT of advice to the contrary, my particular iMac has a video card slot that physically appears to accept 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...

Problem

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.

Help

I am now facing a decision…

1. Do nothing, give up on the project and place my time and energy into something more productive.
2. Sell the NVIDIA GeForce 8800 GS and buy an ATI Radeon HD 2600 PRO.
3. Keep the NVIDIA GeForce 8800 GS and buy a heatsink to match that card.

I aknowledge that there may be other issues with this proposed conversion that I don’t understand, such as video card BIOS and EFi. Advice on how to overcome or avoid these issues in particular would be appreciated.