This is a part of my How to choose a Graphic card guide

A dedicated Graphic card will be connected to a computer using any one of these interface/slot

-->PCI
-->AGP
-->PCI-X
-->PCI-E
-->PCI-E2
-->PCI-E3

PCI ('Peripheral Component Interconnect )
It is an interface or a slot to connect pheripheral components such as internal modems and video cards . A PCI slot can be used to connect any type of component made for it .It is present in all new and older motherboards . Older Graphic cards were connected to the motherboard using this slot .

AGP (Accelerated Graphics Port)
AGP slot can be thought of as a specialized PCI slot made especially for graphic cards .Its faster than a PCI slot .
This kind of slot is generally present in older Motherboards .
AGP is a high-bandwidth interface/slot designed specifically for graphics cards.AGP has gone through three major revisions, with the newest being AGP 8x at 2.1 GB/s .

AGP is now an outdated technology found only in older motherboards

Example : Intel 865 chipset motherboard has an AGP 8x slot . Nvidia 4 series is AGP slot card .

PCI-X (Pheripheral Component Interconnect-Extended)
A Transistion technology that has been outdated and Phased out .

PCI-E (Peripheral Component Interconnect - Express)
It is found in current motherboards . It uses a new technology to give a faster speed than AGP slots . It is what you can call "current technology" .
But it ll soon be outdated by PCI-E2 technology

Example : Intel Motherboards from 915 Chipset onwards use PCI-E slots and not AGP slots .

Example : Intel 945 Chipset has a PCI-E slot , Nvidia 7 series is PCI-E cards

PCI-E2
Found in newer Motherboards . Released on 2007 .

Ex: Some of the 8 series cards are PCI-E2 slot cards

PCI-E3
Slated to be released on 2010 .

A Motherboard will normally contain slots like these

Older Motherboards :- 2 or more PCI slots and 1 AGP slot

Current Motherboards :- 2 or more PCI slots and 1 or more PCI-E slot

Newer Motherboards :- 2 or more PCI slots and 1 or more PCI-E2 slot

NVIDIA SLI and ATI Crossfire motherboards will have multiple PCI-E slots and also SLI Bridges

A Motherboard with PCI and PCI-E slots