TweakTown has posted a comparison between XFX GeForce 8800 Graphics Cards

Most these differences can be seen on the front of the card. With the coolers being slightly larger in size on the GTX, the only other difference between them is that the model number differs. Clearly we can also see that when it comes to size, the GTX is much longer than the GTS – it comes in at 10.5 inches long. nVidia say that they did their research and enthusiast uses most of the time have the case to carry a card so long but if you’re looking at the GTX, we urge you to make sure you can before buying as it is really long. There are also some differences when it comes to the PCB layout of the part of the card that we can see.

XFX GeForce 8800 Graphics Cards - GTS & GTX Compared