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AGP - Accelerated Graphics Port:

AGP Technology provides a dedicated, High Speed Graphics Port Interface / Platform Bus Specification from Intel through which large blocks of 3D texture data can be moved between the Computer's Graphics Controller (Display Adapter) and System Memory.

AGP is faster than PCI. PCI has 33MHz / 32 bit PCI Bus & 66MHz / 64 bit PCI Bus Slots both supporting Bus Mastering & IRQ Sharing. Currently only Server MotherBoards have 66MHz / 64 bit PCI Bus Slots. Bus Mastering allows communication to occur between Device & Memory without CPU intervention. CPU accesses to System Memory proceed concurrently with AGP Memory reads by the AGP Graphics Controller. This allows the CPU to perform other essential tasks.

extremetech_linksmaster - Video Charts: Videocard Performance Charts - Next Generation Models from AGP to PCI Express.
Matrox Graphics - News - CAD and GIS: Matrox AGP (introduced by Intel in 1997), PCI, and PCIe guide - Matrox guide to different types of expansion slots and add-in cards

AGP GART Support (Graphics Address Remapping Table) - "During AGP memory initialization, the OS allocates 4K byte pages of AGP Memory in main (physical) Memory. These pages are usually discontiguous. However, the Graphics Controller needs Contiguous Memory. A translation mechanism called the GART (Graphics Address Remapping Table), makes discontiguous memory appear as contiguous memory by translating virtual addresses into physical addresses in main memory through a remapping table."

www.intel.com - Accelerated Graphics Port Technology Home:
1. AGP Technology Overview
2. www.intel.com - AGP Tutorial Chapter 2 - AGP Memory Allocation:
318023 - Computer Stops or Continually Restarts With ATI RADEON Video Adapter: Using Incorrect Display Driver version.
324764 - Problems When Windows XP Tries to Load the Agp440.sys Service: Changed Video Cards lately? Oops! Possibly using an incompatible Video Driver during WinXP Startup. After disabling this Service from Recovery Console, Update Video Drivers upon reboot.

Only one AGP slot is allowed on AGP equipped MotherBoards. The PCI slot previously used for the Display Adapter can now be used for other PCI Slot Devices. The brown AGP Slot is slightly shorter than the white PCI Slot and is located about an inch farther back. AGP uses a 66MHz / 32-bit Bus architecture. Current Design changes indicate AGP 8x & PCI 2.2 will be replaced with PCI Express. Done! GB - Enjoy!

AGP Specification	AGP Multiplier	Bandwidth	Signaling Voltage	Slot Keying
AGP 1.0	AGP 1x, AGP 2x (original)	266 MB/s, 533 MB/s	3.3V	Type 1 (now obsolete)
AGP 2.0	AGP 4x, AGP 2x	1066 MB/s, 533 MB/s	1.5V	Type 2 (4x/8x compatible)
AGP 3.0	AGP 8x	2133 MB/s	0.8V	Type 2 (4x/8x compatible

Note: Chart above quoted with emphasis on AGP Specification & Slot Type. Chart below quoted according to AGP Name & Bandwidth.

Name	Connection width (bits)	Bits/ cycle/ line	MHz	Voltage (V)	Introduced with…	Potential bandwidth (MB/s)
AGP 1x	32	1	66	3.3 or 1.5	AGP 1.0 (at 3.3 V)	266
AGP 2x	32	2	66	3.3 or 1.5	AGP 1.0 (at 3.3 V)	533
AGP 4x	32	4	66	1.5 or 0.8	AGP 2.0 (at 1.5 V)	1066
AGP 8x	32	8	66	0.8	AGP 3.0	2133

Summary Table above quoted from Matrox.