Had to link to this great post over at PTaylor’s MSDN blog about how video card GPU’s are sorted between the different grades of video cards.
The tl;dr version is that GPUs are created, tested, and sorted by how they perform. In essence, your high-end 8800 Ultra has the same GPU as the low end 8400. Its just that the GPU in the 8400 has imperfections in the manufacturing process. Instead of ditching a malfunctioning chip, they simply reduce the clock speed and shut off different parts until it works. How slow it is and how much stuff is shut off determines into which rank of video card the chip goes.
Interesting stuff. The guy makes the obvious conclusion that your low end video card is just a defective high-end card.