Over the last ten years, wireless networking has become nearly ubiquitous. Every coffee shop, most bars, and quite a few of the grocery stores in modern America feature a wireless network. Wireless has also taken over the home to the point that almost anyone who has broadband Internet probably also has a wireless access point.

USB

The easiest way to attach wireless to a desktop machine is to plug in a USB wireless device. While this is the easiest method, it isn't necessarily the best. USB is inherently slower than any other interface used to attach a wireless card to a computer. Another detriment is the exposed nature of the device. It's not particularly uncommon for a USB device to be snapped out of its socket, likely ruining the device and the port into which it is plugged.

PCI

PCI (Peripheral Component Interconnect) used to be the most common form of internally connecting an add-on card. Even five years ago, it would have been the best method to connect such a card. However, it runs at speeds that by modern computing standards would be considered anemic (though still faster than USB). It also suffers from a lack of available slots. The average motherboard today can have as few as two PCI slots available.

PCI Express

At the moment, PCI Express is the best solution to adding wireless networking to a desktop machine. Typically, networking devices are built as PCI Express x1 cards. This means that they can be inserted into any PCI Express slot, as x1, x4, x8, and x16 slots can all accept an x1 card. The card will run at speeds equivalent to the built-in wireless on a laptop. The only disadvantage to PCI Express is that older machines do not support it.

Built-in

Many computers currently in production have built in wireless. Apple's entire line of desktop machines come with wireless, soldered directly on to the motherboard. Dell, as well as other PC manufacturers, are offering wireless connectivity as a build-to-order option, using PCI Express cards. Some high-end enthusiast motherboards are also available with built-in wireless.

Speeds

The options for wireless add-ons can be quite varied, and often confusing. Not only are the methods of connecting a wireless card quite varied, the type of wireless card is also important. The most common access points are Wireless-G, running at 54mbps. Also available is Wireless-N, with speeds up to 600mbps. While N is faster, compatible access points are less common, limiting it to G class speeds. It's also more expensive, and bulkier, as it uses multiple antennas to achieve its higher speeds.

Source:

Wireless Networking in the Developing World

What is PCI Express?

PCI-SIG - Mini PCI