Peripheral Component Interconnect Express and Peripheral Component Interconnect graphics cards may be exactly the same card model, but use different connection types that have different limits for the card's maximum performance capabilities. The PCIe standard supersedes and offers superior performance to the PCI standard for connecting peripheral cards to a computer. PCI graphics cards are limited by the slower data transfer speed and do not have the graphical potential of PCIe graphics cards.
PCI, AGP and PCIe Graphics Cards
Graphics cards connect to computers through three different connection types: PCI, AGP and PCIe. PCI and PCIe interfaces are intended to support a wide range of devices, while AGP is solely intended for graphics cards. The AGP standard is no longer in production. The AGP standard came in to play when the PCI standard could no longer provide the necessary bandwidth to maximize a graphics card's potential; the PCIe standard did the same thing to AGP as AGP did to PCI. PCIe cards were intended to replace PCI cards.
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PCIe graphics cards communicate with the motherboard at 40 times or greater the rate of PCI graphics cards. The additional bandwidth provided by the PCIe connection type lets graphics cards handle significantly more complicated graphical data, which means PCIe graphics cards are able to produce better visuals than PCI graphics cards. However, it is possible for a high-powered PCI card to produce better graphics than a low-powered PCIe card. PCI graphics card performance is limited by bandwidth, and more powerful cards see diminishing returns when using an interface such as PCI.
PCI and PCIe use different physical connection types. Even though both standards have "Peripheral Component Interconnect" in their names, they are two different technologies. PCI and PCIe cards and ports have physically different connectors. Both PCI and PCIe come in backward- and forward-compatible revisions for their respective port types. PCIe graphics cards can use all PCIe ports that are of the same size or larger, but can't use PCI slots. PCI cards can't use PCIe slots as well. PCIe cards and ports come in multiple sizes to handle different tasks depending on bandwidth needs. PCIe graphics cards typically use the 16-lane version of PCIe with the largest port size. A Wi-Fi adapter may use the smaller one-lane PCIe type.
Same Card, Different Connection
It is possible for the same graphics card model to come in both PCI and PCIe revisions. The PCIe version of the card will always perform as well or better than the PCI version of the card. The PCI versions are intended for computers that do not have PCIe connections. Lower-powered graphics cards may see the same performance level on PCI and PCIe because the graphics card does not require more bandwidth to optimally perform than the PCI standard provides.