here. The computers are close together, and a wireless network will be fast to set up and configure. Having said that....it's fair to point out that a wired network would also be fairly easy to set up - it doesn't sound as if much cable would be required, and you would not have any of the security concerns that can accompany wirless networks.
It's your decision, but in your place I think I would probably opt for a wired network. For that you would need LAN cards in each computer and a 5 port broadband router (assuming you have a broadband conection to share between the machines). You'll also need cables to connect the computers to the router.
If you opt for a wireless network you'll need a wireless ADSL router/modem combination (again assuming there's a broadband connection), plus wireless network adapters (either internal or external) for all the machines.