PCI-Express is replacing AGP, PCI-X is a half-baked idea for replacing the old PCI standard (I guess we'll see PCI-X vidcards, but it'll be a slower thing like current PCI vs. AGP vidcards).
News.com discusses the recent adoption of AGP 8x on NVidia's nForce2 chipset. But dates this standard and expects PCI Express to overtake it in about 1 1/2 years: "AGP has kind of worn out," said ...