This can sometimes be an inexact science.
Within iChat, your buddies will have to have the latest version of AIM, I believe. Obviously they'll have to have a webcam as well, otherwise you'll just initiate a one-way video chat. Also, they can't be webcamming with anyone else, since a video stream can only be sent to one app at a time.
Also, Panther and Tiger could video chat with non-Mac users... there was no particular need to upgrade to Leopard to accomplish this.