Can I replace/upgrade 4850 graphic card in 2009 iMac

"Yes, you can ""replace"" the 4850 card in a late 2009 iMac 27"", but I preferred to fix mine, which you can see in my rather comprehensive video here:

https://youtu.be/v1DXBLkiqFM"

"This is an old question, but I thought I would add. I was able to do exactly what you asked in my late-2009 27” iMac. I found a Radeon HD 6770M 512MB GPU that was pulled from a 2011 model on eBay. It dropped right in and since it came from an iMac it woked just fine.

The only big issue is that I am running El Capitan and I was not able to upgrade the OS (I forget if it was Sierra or High SIerra). It would error. There seems to be a conflict with the installer and the graphics drivers. I’m sure it has to do with the non-standard card that is in the machine now. I am happy to just run El Cap until the machine dies on me, but keep this in mind."

"Yes you can replace your graphics card with the late 2009 iMac's Ati 4850 or newer Ati 5750 used on mid 2010 iMacs.

older Ati 4850 has 512MB of Ram and 256Bit Memory Bus GDDR3

newer Ati 5750 has 1GB of Ram and 128Bit Memory Bus GDDR5

It is your choice, but personally i don't prefer downgrade your memory bus speed to increase amount of ram. Bus speed is more important than ram size on older graphics cards."

I purchased an iMac 2009 with a half-dead HD4850. After a failed re-baking, I gave up the HD4850 and install a Quadro K1100m 2GB instead. It has been working beautifully since then.