Answer to: Backlight dim in a part of the screenIt is not a graphic issue. What's a strange solution? It is definitely not a graphic card. Of course, it is possible to fix it but it is very hard. 1. In all cases that I had only the left side of the screen was not working. 2. When I dissembled it I noticed that it faulty lcd panel (factory defect). I can explain what I mean. It appeared that all pins of the plug-in on the left side of the backlight lamp are not in the right position. For example, pin 1 on the place of pin 2, pin on the place of pin 3 and so one. 3. That's why, in order to repair it you should dissemble all screen and solder backlight plug-in anew. And it is very hard. I have spent 3 hours to repair it. And you also should be very careful not to damage lcd screen and other parts of the screen. Next time I can make tutorial how to fix it if someone still need it.
Answer to: Can I upgrade the graphics card on my iMac?Yes it is possible to use 4850 graphic card but the problem that it is not possible to place it inside because 4670 is much smaller, but you can upgrade to 5670 and even to 6750 graphic card from imac 21,5 2011. In these case the size of these cards is the same. The same situation with iMac 27 Core2Duo with Ati 4670 graphic card, but there you can use 4850, 5670 and 6750 and 6770 graphic card