I think iChat is the most efficient out there in terms of picture quality-for-bandwidth. Yahoo's quality is unspeakably awful. If iChat doesn't work well, your alternative is prettymuch super-jerky or super-blurry video, neither of which have a whole lot of point.
You could try Skype. The video will probably be worse but it has much nicer audio quality than iChat so at least you can carry on a good conversation...