In the new Messages App you can have an iMessages account and older "iChat" type accounts
iMessages accounst are created with registering an Apple ID which can also be used on iPhone and iPads or iPod touches and iMessages are then synced across all devices.
It is an Apple Only service and is text only
In the Main Message Window/interface once you have starts an iMessage chat you can use the Video icon which will start up FaceTime to Video Chat
"iChat" Type accounts
Messages in built on top of what was iChat and you can still use the Names and IDs that you used in iChat.
You can see lowere left in the pic I have 10 such accounts logged in (The pic also indicates I keep the Buddy Lists separate - if you use the combined list the view is different).
You can then work two ways
Starting a chat like the iMessages way but to an AIM Buddy (or Jabber) and than also use the Video icon and this will start an A/V chat in Messages.
Example pic of a chat between two of those Logins
Here the Video icon will start an "iChat" Style chat (if it were an outside Buddy - obviously I can't video to myself on the same computer)
The alternative is to have the Buddy lists On show and work from a Buddy list and it's icons
This is the same Account as the above pic but in the Buddy list.
You can start from the Green icons or highlight a Buddy and use the bottom icons (or the Buddies Menu) or right click a Buddy.
To Unlink Combined Buddy list go to Messages Menu > Preferences > General Section and untick the line about Gathering all Accounts into one list (It's actually all Non iMessages accounts as the iMessages account does not have a Buddy list)
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
iMac 2.5Ghz 5i 2011 (Mountain Lion 10.8.2)
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
"Limit the Logs to the Bits above Binary Images." No, Seriously