The best method would be to use FaceTime, but because you are not using the latest versions of OS X, then the account you might want to get is to get an AIM, Google Talk, Yahoo messenger accounts to be able to video chat between the two iMac’s running Mac OS X10.5 and 10.6. Check out the article below for more information about iChat and the different accounts that can be used with it. Keep in mind that because you are using different versions of OS X that not all account will show up to be seen.
AIM or Jabber.
AIM valid accounts include any @mac.com ID (Lapsed, paid for (originally) or a Trial account) Any @me.com account that was linked to iCloud at the change to iCloud from MobileMe. And of course any @icloud.com ID.
If you have an Apple ID that started as one of these and "Migrated" to the other services other the years and is linked for Email they will work as separate IDs/Valid Screen Names with the AIM servers.
Effectively this then works Mac to Mac in Messages or iChat.
Many servers http://www.jabberes.org/servers/ (this is only some and the site also list many that are Off line)
Also Google run what was called the GoogleTalk server which means Google Mail IDs can be used as Jabber IDs
Facebook run a separate Chat ID/Server which you have to sign up for at Facebook.
AS most server are what is called "Federated" you can add Jabber Buddies form one server to a Buddy List of another.
Therefore you may have a Google Buddy list as such but you can add Facebook Buddies or other Buddies on other Jabber servers.
Audio and Video Chat are Mac to Mac. You cannot Video or Audio chat to Web Browser login even if the Browser has the video plugin for that service (Basically web browser to web browser chat require the Video plugin for that service/server).
FaceTime will not work.
You can download FaceTime for the Snow Leopard Computer as it is an Intel only app.
However it cannot be downloaded for the Leopard computer.
You may have IDs that will work in iChat 4 and 5 already (Apple IDs as AIM Names or Googlemail or Facebook IDs)
Chats are then Mac to Mac (and also AIM to AIM or Jabber to Jabber but not AIM to Jabber).
iChat will allow you also to do Screen Sharing as well as Audio Only chats
10:05 pm Saturday; August 9, 2014 iMac 2.5Ghz i5 2011 (Mavericks 10.9)
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb (Snow Leopard 10.6.8)
Mac OS X (10.6.8),
Couple of iPhones and an iPad