Hi,
As Dick M says you each have to have an AIM valid Name or both have Google or other Jabber IDs.
Most of the big IM services now seem to offer at least Text messaging from one service to another.
This is done with a backend link at the server level.
iChat will Video chat to any Google ID (Or other Jabber ID) as long as the other person is also using iChat.
This is because no matter what Buddy List you start from iChat uses the same connection Protocols (Called SIP = Session Initiation Protocol).
Jabber servers use a Protocol called Jingle.
Google's variant of this is not that compatible with other Jabber apps that have the Jingle Library included.
You with a Google ID cannot Video to anyone Logged in a Web Browser to the Google Mail Web Page and having the Intel Only Mac Plug-in for Browsers or the PC one.
Nor to the Stand-alone PC app called Google Talk
Or anyone using a Jabber app whether on a Mac or on a PC.
(if you do have an Intel Mac and get th Plug-in for Browsers you are likley to be limited to Other Google Buddies whether also logged in in Web Mail or using the Stand-alone PC app).
On the AIM Side iChat will Video Chat to any Version of iChat which has an AIM valid Name.
These can be from AIM or the @Mac.com and MobileMe names are also valid with the aIm servers (Passwords need to be 16 characters of less for the AIM servers)
It will also Video chat to Trillian Pro on a PC (If they have an AIM name) or the AIM app for PCs (Strangely less reliable than Trillian).
Lion, due in July, is going to add Yahoo to iChat but most likely this will be at the same level as the Google/Jabber option is currently
(you are unlikely to be able to Video from a Yahoo list to a Google/Jabber or AIM list)

8:58 PM Thursday; June 30, 2011
Please, if posting Logs, do not post any Log info after the line "Binary Images for iChat"
G4/1GhzDual MDD (Leopard 10.5.8)
MacBookPro 2Gb( 10.6.8)
Mac OS X (10.6.7),
"Limit the Logs to the Bits above Binary Images." No, Seriously