pci slots are for connecting things like sound cards, capture cards, modems, ethernet cards, and even video cards..
however, pci slots can severely bottleneck a videocard's ability to communicate with the rest of the computer.. so agp slots to the rescue!
"accelerated graphics port" (lol.. i hope thats right ><)
agp ports (there are 2x 4x and 8x) allow for more advanced video cards to be used without the bottlenecking involved with pci slots.. agp slots are a bit old though... so to make way for the future video cards, they introduced the pci-express interface.. more bandwidth etc.. agp slots are still good though, they don't bottleneck even current high end video cards (at least i don't think..) but companies are moving towards the future and all the great cards currently out are only for the pci-express. there are still some good video cards for the agp, but not a lot.. the only agp cards that are better than the 6600gt would be something like 6800gs, 6800gt, x850xt, and the 7800gs. if you can get your hands on the gainward 7800gs.. thast actually a 7900gt in disguise!
you CAN NOT use pci-express cards in pci slots (and vice versa)
i hope my rambling made some sense to you...
"Is virtue a thing remote? I wish to be virtuous, and lo! Virtue is at hand." -Confucius