I guess that I don't have to explain too much about AGP, the 'accelerated graphics port', anymore, which is nowadays the state-of-the-art interface between the system chipset and the graphics card. AGP was developed three years ago to allow data transfer between the system and the graphics adapter at a significantly higher bandwidth than PCI. It was 1997 when graphics cards with 3D-acceleration became not only fashionable, but also pretty common, and those 3D-accelerating graphics cards required much more data from the CPU and memory than their only '2D-accelerating' predecessors. AGP was born to accommodate those new needs.
The Theory of AGP-Performance vs. PCI-Performance
Already at the first launch of the AGP, specifications allowed two different AGP-speeds, AGP1x and AGP2x. The main differences between AGP and PCI start with the fact that AGP is a 'port', which means it can only host one client and this client ought to be a graphics accelerator. PCI is a bus, and it can serve several different kinds of clients, may it be a graphics accelerator, a network card, a SCSI host adapter or a sound card. All those different clients have to share the PCI-bus and its bandwidth, while AGP offers one direct connection to the chipset and from there to the memory, the CPU or the PCI-bus.
The normal PCI-bus is 32-bit wide and is clocked at 33 MHz. Thus it can offer a maximum bandwidth of 33 * 4 byte/s = 133 MB/s. The new PCI64/66-specification offers four times as much, it comes with 64-bit width and a 66 MHz clock, thus its bandwidth limitation lies at 533 MB/s. However, please let's not forget that PCI64/66 is hardly supported anywhere yet and it was particularly developed to host I/O-controllers with very high data bandwidth, as e.g. IEEE1394 or Gbit-network interface cards. AGP is clocked at 66 MHz to begin with and it is 32-bit wide. This offers a maximal bandwidth of 266 MB/s in case of AGP1x, where data is transferred the common way, at the 'falling edge' of each clock. AGP2x offers 533 MB/s, by transferring data at the rising as well as the falling edge of the clock. The new addition called AGP4x doubles this bandwidth another time to 1066 MB/s.