Why use the AIM client when iChat works better for most things, uses the AIM system (that is, you can see and talk to other AIM users) and certainly is good for video chatting. You can sign up for as many AIM screen names as you want through the AOL AIM site (
http://dashboard.aim.com/aim ); sign up for two (it's possible you'll need to give them a second email address, but if you don't have two you use just sign up for a free one from Yahoo or Gmail or whoever).
Once you've got two AIM screen names, plug one into the iChat installed on each Mac, add the other to the address book on each, and you're basically good to go--if you open iChat on both the other screen name should show up in the Buddy List, and it's just a matter of clicking the camera icon to initiate a video chat.
There's also Skype, but iChat is probably easier to work with since it's already installed on both computers (and, assuming you have at least Tiger on both machines, the video quality should be higher as well).