In the iMac 24 ", the video card is not soldered to the motherboard, but it connected via a MXM connector.
In theory it should be possible to replace it, but in practice Apple does not offer (to install) a new card, and they are rare in the PC world.
The situation may soon change.
Asus has announced a unique product. It is a PCI-Express card carrying 3 HD Radeon 3850's, these cards are connected via MXM.
Of course, there would still many obstacles in order to install one of these daughter cards in an iMac, but it would be really simple if Apple released a Radeon 38xx HD for the Mac Pro. Then we would have a compatible or least adaptable firmware.
[translation by jeremy]
