"AGP 4x is only capable of transferring data at a maximum bandwidth of 1.06GB/s. Compared with all the existing buses in the system, it is rather low and 3D performance would bottleneck at the AGP port. There's really no point in having kickass technology like HyperTransport if the AGP is limiting data transfers at 266MHz (4 x 66MHz). Now that AGP bandwidth is doubled to 8x, the clock frequency will increase to 533MHz which means that it will transfer at about 2.1GB/s. "

The AGP 3.0 standard is also known as AGP 8x.

  transfer (maximum) / Bus Bandwidth clock rate   bytes per transfer
AGP4X 1.06 GB / second 266Mhz 266.67MHz
(4 x 66 MHz)
4 (32 bits)
AGP8X 2.10 GB / second 533MHz 533.33MHz 4 (32 bits)

more comparison details

NVIDIA GeForce4 Ti 4200 with AGP8x (NV28)
NVIDIA GeForce4 MX 440 with AGP8x (NV18)

the NVIDIA GeForce FX 5200 also supports AGP 8X