You might want to do a little cleaning, just recently, my ATI Radeon X1900 XTX in my pc was overheating, and acting up. I dissasembled the heatsink for the card, and cleaned the dust out of it (and there was quite alot of it, since i've had it for 2 years) and after that, removed the heatsink entireley to re-apply the thermal paste(i used artcic silver 5, the best stuff around) and not the card runs like new, the graphics card heatsink should be removable, and if your warrenty is expired, apple can't really help, so if you want to take apart your iMac, try cleaning out the dust, from all of the heatsinks (even the cpu) and then remove them and re-apply a thin layer of artic silver 5 (a tube costs about $10 at radioshack) and then put it back together.
Good Luck,
Chris