Answer to "Backlight dim in a part of the screen"It 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 Radeon HD 4850 graphics card but the problem that it is not possible to fit it inside because 4670 is physically smaller and there just isn’t enough room for it and the larger heat sink. But, you can upgrade to Radeon 5670 and even to 6750 graphics card from a 2011 21.5” iMac. With these the size of these cards & heatsink is the same. The same situation with 27” iMac Core2Duo which have the Radeon 4670 graphics card, there you can use these Radeon cards: 4850, 5670, 6750 and 6770.