the boot screen only stops showing the apple startup one windows /osx takes over you have a display ....osx does support and cards i have a 7950 in my mac pro and i have a boot screen (flashed bios) i have yet to find a recent pc card from nvidia that you can just flash a bios onto tho windows is installable without a bootscreen as is osx if you need to ......you can start the iMac in windows using sys prefs / start up disc without the need for a boot screen and use bootcamp in windows to restart in osx