You wont want to do either WPA. What you want to do is 1 of 2 things:
1. Setup using WEP and use a GOOD password. Nothing simple! I may help to do something like put an * or $ in front or at the end of the password. This makes it a little harder to crack.
2. Setup MAC Address Filtering. MAC has nothing to do with Macintosh. Its a network term. In the computer world, every network device has whats called a MAC Address, from an LPT1 port to an Ethernet Port. Its just unique address mapped to that device. What this will do is it will ONLY allow computers with the MAC addresses you put into the AirPort as computer with allowed access to have access to your network. So it doesn't matter if they can see the network, they cant connect to it because its not on the list of allowed MAC addresses. Another thing you can do further is create a closed network. This will tell the Airport to not broadcast the signal. So to see the network you will have to know exactly what your Airport is called.
MAC Address is the most secure type of filtering you can do. WEP can be broken sometimes easily. MAC Address can be broken by someone trying to spoof your MAC Address, but its more work that its worth so most people will just go on to something else.
You can enable MAC Address Filtering inside the AirPort Admin Utility. Under the Access Control Tab, you will see something that says AirPort ID and Description. To the right of that is a + and - buttons. Click the + and enter the AirPort ID (aka MAC Address) of the computers you want allow network access. Give a description of what computer is what so you know what MAC Address each computer is if you ever need to take one out quickly. You can get the AirPort ID by going to system preferences on that Mac and then click on Network and then click on Airport and it will tell the AirPort ID there. It will be a number letter combination with a : in between each 2 numbers/letters. (ie. 33:a0:8d:99:45:e3).
If you also want to make it a closed network, while in the AirPort Admin Utility, click on the AirPort Tab and just under the AirPort Name is a checkbox that says Create a closed network. If you check that box the AirPort will no longer broadcast the AirPort name to the outside world. So in order to join it, you will have to know exactly (including caps) what the name of your Airport is. Then join it by going to the AirPort waves menu bar by the clock and clicking on Other... Then enter exactly what the name of your AirPort base station is and it should join as long as you typed in the MAC Address correctly.
I hope this helps. If you live near a lot of neighbors or in a busy neighborhood its a very good idea to do something to secure yourself. With access to your network people can not only browse the internet using your connection, but also gain access to your files, printers, other computers on the network, etc if they know what they're doing. And they can be around the corner out of your site when doing so and still possibly get a wireless signal from your AirPort.
