Key Difference: AGP stands for Accelerated Graphics Port. PCI Express stands for Peripheral Component Interconnect. PCI Express card is an advanced version of the PCI graphics card. AGP cards and PCI cards are placed in certain specific slots only. AGP cards are slower than PCI express cards. They primarily differ in the connector configuration.

A graphic card is a component that is linked to the motherboard of the computer. It uses the digital information to form images. These cards fit into the specific slots on the motherboard. It depends upon the slot supported by the motherboard on the basis of which a particular type of card is used.

AGP stands for Accelerated Graphics Port. It is an interface specification that has been designed especially for the 3-D graphics; focusing on a faster and smoother display. This specification has been developed by the Intel. AGP (Accelerated Graphics Port) was the commonly used type of graphic card during the 2000s. Later, it got replaced by the PCI express card.  AGP cards are attached to the main board in a particular slot; with some extra high data transfer rate. The interface has been designed in a way that it keeps the graphic card and PCI bus quiet apart. AGP 3.0 video cards allow backwards compatibility with AGP 2.0 (4X) motherboards however, the card works according to AGP 2.0 specifications, not 3.0. The card is also equipped with a graphic chip. It carries a very fast DDR RAM for dealing with textures and 3D data. The slots are generally brown in color.

PCI Express stands for Peripheral Component Interconnect Express. PCI Express (PCIe) is a relatively new standard for connecting devices to computers. It can be considered as an upgraded version of PCI as it is capable of providing higher potential bandwidth than PCI or even more than AGP. A PCIe card fits into a slot of its physical size or bigger. However, it does not fit into a smaller PCIe slot. PCI Express is much faster than PCI. These cards create better visuals than a PCI or AGP card. The same card model can occur in both the forms (PCI and PCI Express).

AGP and PCE Express cards primarily differ in the connector configuration as in the AGP standard, a dedicated, point-to-point interface transmits the data from the graphics card to the system. On the other hand, the PCI Express approach of data transfer is based on the collection of two-way, serial connections. These connections transfer the data in the form of packets. Therefore, PCI express functioning is also considered to be the same as a network connection.  

Comparison between AGP and PCI Express Graphic Card:

Images Courtesy: tomshardware.com