From OCAU Wiki
The AGP (Accelerated Graphics Port) Slot is a slot designed to house a Video Card, as a Video Card will easily saturate the PCI Bus. This slot is becoming less used due to the introduction of PCI-E Slots, which provide much more bandwidth.
There are several types of AGP Slot. Some are compatible with each other whilst some are not.
533MHz clock rate, 32 bytes/clock, Bandwidth: 2.1GB/s [0.8V signal swing}
266MHz clock rate, 16 bytes/clock, Bandwidth: 1066MB/s [1.5V signal swing]
133MHz clock rate, 8 bytes/clock, Bandwidth: 533MB/s [3.3V or 1.5V signal swing]
66MHz clock rate, 8 bytes/clock, Bandwidth: 266MB/s [3.3V or 1.5V signal swing]