PCI stands for Peripheral Component Interconnect. It was developed by Intel in the early 1990s and designed to replace the slower EISA bus and VESA Local Bus (VLB). PCI was developed to be more compact and to provide more available motherboard slots than VLB, and it provides much faster data-transfer speeds than the older buses.
PCI Basics
The PCI bus has a theoretical maximum of 133 megabytes per second (MB/s). This allows more complex instructions for your video display card and sound card. EISA and VLB can't typically allow enough speed to provide the three-dimensional audio effects that come with multichannel sound.
Complex 3D video operations such as occlusion culling (not drawing the parts of objects that the viewer cannot see), overlapping transparency effects, motion blur, and realistic shadows are generally not possible with a VLB or EISA video card. The older cards are also limited in the screen resolutions at which they can render 3D and 2D environments. Some older PCI cards have a cap of 33 MB/s or 66 MB/s.
PCI-X
There is also the PCI-X hardware standard that comes in 266 MB/s and 533 MB/s flavors, as well as 133 MB/s. It was developed by IBM, HP and Compaq. PCI-X is 64-bit, meaning that the processor chip can "see" 2^64 bits of total memory at any given moment. Likewise, "32-bit" means a processor can see 2^32 bits, which corresponds to 4 gigabytes (GBs). PCI-X was designed for file servers and Web servers that had more than 4 GBs of memory installed.
PCI Express
Both PCI and PCI-X were replaced by PCI Express (PCIe) in 2004. PCIe uses a serial interface, while earlier PCI implementations used a parallel bus. With PCIe, multiple devices can send data through their channels to a central processor simultaneously. A device can also be granted multiple channels. PCIe slots are also designed to accommodate multiple video cards.
SLI and CrossFire
nVidia's version of this is called "SLI," which stands for Scalable Link Interface. ATI's version is called CrossFire. Here, the two cards can communicate to each other directly, instead of having to use the processor channels. They can alternate what frames they want to render, or split the work of a single frame 50/50.
Think of them as two video cameras working in tandem to not just record a scene, but to draw everything in it, ideally no slower than 30 frames per second. Furthermore, a video card can contain multiple graphics processors, speeding up the frame rendering process considerably.
ExpressCard
Your laptop may also have external PCIe slots, known commercially as "ExpressCard." This technology replaces CardBus. CardBus has a maximum speed of about 1 gigabyte per second (GB/s), which is shared with your other CardBus devices. So with two of those plugged in and pushing their maximum speeds, they'll be limited to 500 MB/s. ExpressCard, meanwhile, offers up to 2.5 GB/s for each connected device. ExpressCard slots are not compatible with Cardbus devices, so many manufacturers provide slots for both.
Tags: older cards, PCIe slots, video card