A wide variety of PC cards for attaching external devices or extending functionality to a computer mainboard are available. This Data Sheet provides a brief description of each PC card standard.

PCI
PCI stands for Peripheral Component Interconnect, which specifies a computer bus (slot) for attaching peripheral devices and adding functionality to a computer mainboard. With PCI plug and play support, the host system OS automatically configures settings of a PCI card inserted into a mainboard PCI slot. PCI cards are available in 32-bit and 64-bit bus widths. 32-bit PCI slots and cards are typically provided personal computers, while 64-bit PCI slots and cards are employed in workstations and servers. PCI bandwidths are provided in the following table:

Frequency & Width	32-bit PCI	64-bit PCI
33 MHz	133.3 MB/sec	266.7 MB/sec
66 MHz	266.7 MB/sec	533.3 MB/sec

PCI-X
PCI-X stands for Peripheral Component Interconnect Extended, a computer bus and expansion card standard designed to supersede PCI. Essentially, PCI-X is a faster version of PCI. PCI-X offers 64-bit (double) bandwidth, revised protocol, and increased maximum signaling frequency (133 MHz). PCI-X is generally backward compatible with PCI, meaning that, a PCI-X card can be installed in a PCI slot provided it has the correct voltage keying for the slot and (if inserting in a 32 bit slot) nothing obstructs the overhanging part of the edge connector. PCI-X standards are detailed in the following table:

Standard	Categories	Voltage (V)	Frequency (MHz)	Bandwidth (MB/sec)	Maximum Number of Slots
PCI-X 1.0	PCI-X 66	3.3	66	533	4
	PCI-X 133		100	800	2
			133	1,066	1
PCI-X 2.0	PCI-X 266	1.5	266	2,133	1
	PCI-X 533		533	4,266	1
PCI-X 3.0	PCI-X 1066	1.5	1,066	8,533	1
	PCI-X 2133		2,133	17,066	1

PCI-Express (PCI-E, PCIe)
PCI-Express is a flexible system intended to replace PCI, and PCI-X. The official abbreviation PCIe, or PCI-E is frequently used. PCI Express is referred to 3GIO, or 3rd Generation I/O. PCI-E is physically and electrically incompatible with PCI or PCI-X cards. PCI-E provides faster data transfer at lower voltage. PCI Express 1.1 transfers data at 250 MB/s in each direction per lane. The number of lanes can be customized to x 1, x 2, x 4, x 8, x 16, and x 32. With a maximum of 32 lanes (PCI Express x 32), the PCI Express allows for a total combined bandwidth of 8 GB/s in each direction. PCI Express is both full duplex and point-to-point. This means that although both the PCI-X 133MHz 64-bit and the PCI-Express x 4 (four lane) will offer similar level of performance if data is being accessed/transferred in one direction inside a single device, the PCI-Express x4 provides a better performance even if multiple devices are being used at the same time or if the communications inside a single device is bi-directional.




ExpressCard
The ExpressCard standard, leveraging PCMCIA PC Card and CarBus technology, brings high performance I/O to the next generation of personal computing devices. Newer notebook and desktop computers are increasingly replacing the conventional PC card slot with an ExpressCard slot. The smaller form factor ExpressCard provides economical expansion opportunities for external devices.
The ExpressCard provides the following major benefits:

Reduces footprint with smaller form factor design

Supports hot-plug and auto configuration, easy to use

Provides reliable, durable, and expandable flexibilities

Conserves resources with efficient power management

Two sizes ExpressCard modules are provided in two standard formats, the ExpressCard/34 module (34mm x 75mm) and the ExpressCard/54 module (54mm x 75mm). Both formats are 5 mm thick and use the same connector interface. The two ExpressCard module sizes provide system manufacturers greater flexibility in development of ExpressCard compliant devices. The ExpressCard/34 device is more suitable for smaller systems, while the ExpressCard/54 one is more accommodating for applications that do not physically fit into the narrower ExpressCard/34 format. Thus, the 2-port eSATA to ExpressCard host adapters under ExpressCard/34 module are perfect for compact systems!



*Reference: http://en.wikipedia.org/; http://www.expresscard.org; http://www.tomshardware.com