Here's the information on recording and saving video chats from Apple's Help files:
To record a video chat:
Start a video chat with one or more buddies.
Choose Video > Record Chat.
To stop recording, either click the Stop button, or close the video chat window.
By default, recorded video chats are stored in the iChats folder, which is in the Documents folder. To choose a different location for saved recordings, use the iChat General preferences.
You can use Spotlight to search for saved recorded chats by title and date.
I hope that information helps you find the chat.
Claire