Has anyone considered the possibility of using Nvidia drivers from one of the OSX86 projects to make some of these cards work without flashing the BIOS with EFI? The list of compatible cards is here: http://www.tonymacx86.com/wiki/index.php/Graphics_Card_Database
In my mind the process would be to first install the necessary kexts (kernel extensions) for the Nvidia support, and then perform surgery on your iMac to install the new card (an MXM HE card, of course).
I have an iMac 24" Early 2008 with an Nvidia 8800gs and I am considering trying this to allow me to install a card with more RAM so that I can run GPU-enabled Adobe apps such as Premiere which require at least 768mb RAM.