Personally I would contact ATI first and ask them about GDDR3 vs GDDR5 iMac graphic cards for those model before even messing around or wasting you time and money. Normally you can't just yank a graphics card just like machead3 stated. If it where me .. I would backup you iMac HD and one of two things either purchase a used 2011 iMac w the GDDR5 or wait until Apple come out with a new 2012 model. If its true graphics power you want then Mac Pro (towers models) are the best way to go just because of the PCI Express 2.0 slots (16 lane and 4 lane) which what the graphic pros and gamers really need! SSD drives really help a lot also. Processors play a huge part in the graphics cards too .. they have to match the correct specs and data paths to the speeds used to drive each other because of heat issues. ;) Best of luck ~Xtech

Resetting the logic board and testing and replacing the PRAM battery would have been my first step. Then I would have made sure the firmware was updated and properly updated and rebooted on restart. I am a certified Apple tech and found out a lot of people don't know about other good troubleshooting steps involved for proper diagnostic fixes. Yanking and resetting the RAM along with zapping the PRAM is so a great way to resolve some issues. When it come to problems with the logic board more in depth troubleshooting steps are taken. Best of luck with your next Mac.