I relocated recently, and before I left, I shipped a brand new PC with a Sapphire Radeon HD 6970 in the PCIx16 slot across the world. You will notice that the card is a beast. When it arrived, I turned it on to find that the graphics were glitching all over the place. I had to reseat the card multiple times in the slot, and then not bump the chassis to make sure that it didn't lose contact with the slot. Even now, it is wobbling and unstable and I am not sure if it's permanently damaged a new motherboard or not. Either way, my next card will be much smaller, and that new card will come sooner rather than later at this rate.
These things are getting too huge, period. They are out-of-spec with what motherboards, mounted vertically, are capable of handling.