Like Geofelt said, go for either a PCI Express card or a long ethernet cable. My office PC uses a usb dongle I had laying around that I got from big lots for around $10. It overheats very quickly and it gets so hot to the point where it loses connection and I have to remove it and let it cool off before putting it in again.
Here's a good wireless network adapter if you choose to go that route:http://pcpartpicker.com/part/tp-link-wireless-network-card-tlwn881nd
Also many motherboards with built in wifi are overpriced and it would be better to get an adapter or cable instead.