Réponse à : « Can I replace/upgrade 4850 graphic card in 2009 iMac »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.
Réponse à : « Target Display Mode not working. Is thunderbolt the problem? »Dan's answer is correct, they can work together only with DP Mini to DP Mini cable. I have tried both thunderbolt and DP Mini and only DP Mini worked.