Well this question has already been answered a few times before, but I'm nice so I'll answer it again. 2 8800GTX cards are slightly faster than 1 GTX 280. Check the offical reviews thread as there should be some pictures posted and a few links.
If the card is already good enough for everything then there really isn't a need to upgrade.
EDIT: I'll make it even easier. Here is the review that has the tests in it.