No... don't you understand that the wireless password only pertains to who can log onto your wireless connection? A firewall deals with -- once you ARE logged on and have an internet connection, what people on the outside world can see/connect to/hack on your computer.
If your router does NOT have a firewall built in, then you need to have firewalls on your invididual machines (and even if it does)
To oversimplify:
Computers communicate on the Internet through 'ports', numeric identifiers to which various programs and services will respond. When a request comes in on a particular port, say, port 80, and there is a Web server process running on the machine with port 80 open, then your machine's web server process will respond to WHOEVER it is out there, with a message that says "Yup, I'm open for business". They can proceed to then utilize that port to look for information or hack, depending on what's running and how motivated they are.
There are dozens and hundreds of ports for all different types of programs and services. You don't have a prayer of knowing what's going on in the background. And if a PC with open ports is attached to the internet, then its a matter of minutes or hours until it gets scanned and identified as a hacker target.
The point of a Firewall is that it denies responses to all ports, saying "no entrance at this door", or even better, stealths the ports, so it doesn't even appear that there is a door there to knock on.
Then, firewalls can selectively open certain ports (like filesharing) up to only certain machines or groups of machines, so you can have a functioning network. This, of course, requires you to read the instructions and set it up correctly.
The other thing a good firewall software does, is to monitor OUTgoing requests. So if you have brought spyware into your machine, the firewall should flag and block the attempt the spyware makes to make a connection to the internet without your permission.
So: WEP/WPA has ^$^#all to do with firewall security
You NEED a firewall either on the PC or on the router or both, or your Windows machines will become some of the millions of zombied slave machines that spew out spam and worse.
And lastly: You NEED to do some work to understand this stuff. It's not enough to say "Well $#^^ I don't know what all this means, so I'll just plug it all in and let my machines hang out there naked on the Internet." - besides the probability of your machines being mucked up, you will also be making it worse for the rest of us by making it easy for the bad guys and offering them safe haven in your machines.