Pci-e is faster, but current cards don't even max out agp 8x, so as of now, there isn't much point to pci express' speed, but it does give you better future compatibility, and future cards may need the bandwitdth it offers. Also some pci-e versions of same model cards are made to run faster in pci-e form. Like the 6600gt, in pci-e it's 500 core 1000 memory, but agp it's 500 core and 900 memory, so it is faster in pci-e but only because it's clocked higher.