Lol - ill try to explain it then.
first of all AGP and PCExpress 2 video cards are not compatible. your mother board has to have a slot for it.
now on to the drawing board.
AGP

Max speed - 200km/h
PCI-e

Max Speed - 320km/h
AGP is only 8x while PCI express have 16x, so theoretically PCI express runs faster than AGP.
But the problem is they run on the same road, so there's not much diffirence in terms of speed.
Right now, AGP and PCIexpress runs at roughly the same because both needs to talk with the software like Directx and OpenGL.
So AGP is running close to max speed of 190-200km/hr, while PCI express is running at it's average speed of 200km/h,
other's cant simply keep up.
Imagine the driver is talking too fast and the other guys listening to him couldnt understand. So he has to talk slowly so his teammates could understand. It wont be long before we change the race tracks, Soon PCI express will dominate in terms of performance.
And by the way, manufacturers are slowly kiling AGPs, this is ancient technology and they all want to switch to PCI express.