For a card that really depends on what free slots you have at the motherboard, but PCI is very common and most mothersboards can accomodate a few cards.
The alternative is a USB-dongle or an antenna that you connect to the Ethernet port. For these you don't have to open the case.
Be sure that whatever you buy comes with the drivers necessary for your OS, or at least with a download link in the documentation.
Kees
Hello, I'm using a Gateway SX2110G-UW308 desktop pc and I need to find a compatible WiFi card for it. Does anyone know what card I will need?

Chowhound
Comic Vine
GameFAQs
GameSpot
Giant Bomb
TechRepublic