Actually thats where you are incorrect. The potential is there within the pci-e bus, its just not there within the graphics card yet. Once nvidia and ati start making their cards much more complicated (visually) there will be a greater need for speed in delivering the images, and at that point the 16x speed of the pci-e slot will be required and the 8x slot will be unable to keep up. U'll soon see that they will stop making cards for the AGP (if they already havent) and future endeavors will solely concentrate on pci-e.
Google it to find more. I have done a lot of reading in this area b/c i plan to buy a new pc in the very near future as well.