dude they been out forever before AGP slots came out.
PCI
AGP x2 (like 2 PCI's)
AGP x4 (like 4 PCI's)
AGP x8 (like 8 PCI's)
PCI-E x1 (same as AGP x8)
PCI-E x16 (like 2 AGP x8)
So even with the same chipset, AGP is a ton faster than than PCI and PCI-E x16 is wooping faster.