"Do I need a wireless card for the desktop?" Not if you continue to connect it wired. But if you wish to use wireless you probably need to add a wireless adapter if one wasn't supplied with the desktop. I'd get a PCI wireless adapter.
To configure them start with configuring the router (see manual or quick start which comes with it), setting a wireless channel that doesn't conflict with neighbouring wifi and yet gives a strong signal in your environment. Change the SSID from default (usually the manufacturer's name) to something unique.
Back at the computers use the wireless adapter utilities (or Windows Wireless Connection) to detect your router and try connecting.
If everything works, set wireless security type and password in router and apply settings to each wireless adapter.