The famous post #1 where top level information is stored. Contains tables for GPU choices, installation requirements, exceptions, problems etc. The post is incredible informational dense. Probably overwhelming for new comer even avid electronics/computer DIYer. But it is the most important post you will go back through there to pickup info no less than 50 times :)
Most info in this website is learned starting with post #1, following its links, returning over and over. Finally gleamed info to put into this website to simplying choices for the newbies.
So without further delay. Here is link to post #1 in case you've been hiding under a rock while researching this topic
https://forums.macrumors.com/threads/2011-imac-graphics-card-upgrade.1596614/
Obviously no one can read it all. While its true can ignore most of it. How do you find what is relevent? Here are tricks to quickly focus in to find info.
The main macrumors forum with 20k posts have very senior devs that hang out there and started the whole effort. You can imagine they've answered the same newbie question 100 times now and are tired. This will often lead to quick answers or just ignoring. Can't blame them, full tech support is so time consuming and they still need to do these free dev work haha. This happens to every useful DIY forum I've ever been on so not unusual here. This is the motivation to make this website. Just point to a section which then gives complete answer. Hopefully quick and easy.
When asking a question, its best to provide concise background info to avoid a bunch of posts just to get to through the basic stuff. Here is a basic list to include
Model Identifier : Forget the year. Use the Model Identifier (x,y) in System Reports -> Hardware. This is architectural identifier and very concise.
RAM size + SSD/HDD would be helpful
GPU : What is says in About This Mac
Currently running macOS : Stock or OCLP Installed?
What macOS do you want to go to? If 12,x iMacs, willing to settle for Big Sur without paying $$$ for pricy metal GPUs?
Have upgraded through High Sierra in the past?
Any experience with OCLP?
This post is incredibly dense. After installing GPU cards into 4-5 different iMacs, I still refer to this post and keep finding new critical info. Understandable newbies that need to do just 1 iMac gets overwhelmed. Below is a rough summary on the post is organized and critical parts to read to decide on GPU choices
Mainly 3 generation of GPU cards
AMD GCN v1-3 and GCN v4, nVidia Kepler. These are basically most/all the card choices. There are multiple tables for all 3 of these generations. Spoiler 2 lists the AMD choices. Spoiler 3 list the nVidia choices.
AMD GCN v1-3 have 2 tables.
iMacs using LVDS video cables ( Spoiler: AMD LVDS for iMac9,1 24 inch early 2009 or iMac10,1 21.5 inch late 2009 )
10,x iMacs using eDP video cables ( Spoiler: AMD GCN v1-3 GPU (2012-2015) - no iMac12,x 2011 support )
In each table, there are links to where vBIOS is, which iMac size and MXM slot size it works with, and any special installation notes.
Before AND After each table has IMPORTANT details outlining known issues, exceptions beyond whats listed in the tables. Can't skip these. Unfortunately, there usually aren't many links that points to info source on various issues and require searching through the long thread to find it or may not even be present at all (just original author's experience and knowledge) Searching tips here