The options for a Wireless Network Adapter are either a PCI card or a USB adapter. Installing a PCI card involves opening up the PC and plugging the WiFi adapter into a free PCI slot on the motherboard.
Although a PCI card is probably the preferred option, because USB technology was not designed for networking, in practice as long as you have USB 2.0 ports there shouldn't be a problem with a USB WiFi adapter.
The reason for requiring USB 2.0 is that USB 1.1, with a data transfer speed of 12Mbps, restricts an 802.11g wireless adapter effectively to the 802.11b speed of 11Mbps instead of 54Mbps. This doesn't matter for the relatively slow speed (in networking terms) of a broadband connection, but does become significant for file transfers between computers in a local network. USB 2.0 with a data transfer speed of 480Mbps doesn't restrict the WiFi adapter's performance.
I use a USB wireless adapter so that I can swap it easily between computers on the rare occasions when I use a wireless, rather than a more reliable "wired", network connection.