The first version of AGP, now called AGP 1x, uses a 32-bit bus operating at 66 MHz. This results in a maximum data rate for an AGP 1x slot of 266 megabytes per second. In comparison, a standard 32-bit 33 MHz PCI bus (which can be composed of one or more slots) maxes out at 133 MB/s.
As of 2003, newer versions of AGP increase the transfer rate dramatically from two to eight times. Available versions include AGP 2x, AGP 4x, and AGP 8x. In addition, AGP Pro cards of various types exist. They usually require higher voltages and some take up the space of two cards in a standard computer (though they only connect to one AGP slot).
AGP allows for efficient use of frame buffer memory, thereby helping 2D graphics performance as well. In fact, many RAID systems for "headless" (that is, lacking an attached display) servers plug into the empty AGP slot to take advantage of its increased throughput as opposed to PCI.
AGP provides a coherent memory management design which allows reading scattered data from system memory in rapid bursts. AGP allegedly reduces the overall cost of creating high-end graphics subsystems by using existing system memory. However, general system memory, although cheap, performs much more slowly than dedicated on-card graphics RAM, and both mid-range and high-end graphics cards rely on their own high-speed RAM for performance. Cheap low-end graphics cards with little on-board RAM benefited from AGP early in the life-cycle of the technology, but the lowered cost of memory since about 2000 has led to even low-end cards having 32MB or 64MB of dedicated RAM, and graphics now rarely use system RAM.
AGP usage should phase out by around 2005, since Intel have indicated that their future chipsets (scheduled for introduction at about the time of their "Tejas" CPUs) will replace AGP support with PCI-Express. Nvidia's upcoming NV40 GPU will not support AGP in any way, and ATI's upcoming R420 GPU will only support AGP via additional circuitry.
This article was originally based on material from FOLDOC, used with permission. Update as needed.