Ive seen multiple threads discussing about a broken graphics card on iMac and the result was having to replace the whole logic board because the graphics card is soldered on the logic board. Now I was thinking about this and was wondering why can someone just unsolder the graphics card and install a replacement or a better card(if it wouldn't a nice drop in replacement, I think that one could just use wires to make the connections). I noticed on other discussions on processors that they are touchy and need some kind of calibration for the heat they take and it requires a special non public program if one would replace them and that problem might include the graphics card. But if it is just dab here and there and pick up the old one, put in the new one with a few more dabs, I don't see why it is impossible to change the card. If you have to replace the whole logic board it might be worth it to try replacing the graphics card. You would still have to buy a replacement logic board. But, most likely there is some odd problem that prevents one from doing it, but I can't imagine a little time and patience can't solve that problem.