GPU Upgradeability

I have a Dell XPS 8500 with a Nvidia GTX 640 video card. When I purchased the computer I planned on getting a new video card eventually. I have decided that I would like a GTX 660 but due to the smaller size of the case compared to other cases I am unsure whether I can actually fit the card in. I would like to know if I can fit it in and how tight the fit would be before I purchase it.