When you create new account for iChat, just select Google Talk and use your Google account to sign in.
Please see below screenshot.
But if I am not wrong, you can only use voice and video chat with iChat for the others who are using iChat in other words, people with Mac. You can chat with text by typing though.
Adium is also another good alternative and it supports multiple IM including MSN, Yahoo, Facebook, QQ and more.
If you want a good voice chat, you can choose Skype which is available in Mac.
I second to Lionslayer's advice, you can simply use Gmail in web browser for voice and video chat.
Visit http://www.google.com/chat/video from you Mac and install the plug-in.
Video and voice plug-in
Learn more »