well, if agp is anything like pci-e, then the differences lie in the voltages and front side bus speeds. i know for a fact that pci-e 16x and pci-e 2.0 are different because the voltages for 2.0 are increased from 2.5 to 5 volts, and the front side bus speed is doubled as well.
and what all that equates to is double the speed if i'm not mistaken. so you'll get a bigger bang out of the card in the slot if the card is built for 8x. and that in turn i believe will result in better fps.
i'm not 100% on some of that, but that's my understanding. if anyone else knows more than i do, then please correct me and make sure i'm not giving a false impression.