If there is an Nvidia 8800 in your machine , then that is the only card that will go into it, I have had all of these imacs, they have different fittings for different cards, any upgraded card will require a flash with the relevent EFi, it may be a wise idea to check the size of the bios on the card first as the EFI requires 1meg most cards are fitted with a 512 chip, this is also possible to exchange, but none of this is easy.