How did you buy 5 G4 computers and not be aware that they are equipped with airport card slots? Why don't you tell us what computers you have. I saw no mention of laptops in your first post and that would be something to consider when making a network choice.
But yes all your computers can have airport put in.
In a G4 tower they look like this
In a G4 powerbook they look like this
http://www2a.biglobe.ne.jp/~takuya/images/air3.jpg
If you do not what to use airport cards you can go a third party wireless option. ONE model of Linksys 802.11g card (WAP54G) works in a G4 tower's PCI slot so long as you are running OS X.1 or newer. And some model wireless PC cards work in OS 9 or X.
But since you can do airport cards I would go that route instead. It will make the process easier. Read this as a primer. its a little old but the basics are there.
http://www.macintouch.com/airport.html
Wireless is not really superior to wired networks, they are just more convenient. So long as you are not transferring gigabytes of information from computer to computer on a daily basis I would go wireless. You do not need to buy an Airport basestation though, just get a wireless router and that will be more that adequate.