The problem I see is that you're on a PC while she is on a Mac. If her Mac is relatively new, then it probably has a built in camera. I don't think there are any cross-platform messaging clients that also transmit video. (But I could be wrong.)
One possibility you might explore is bypassing the instant messaging client for the video portion. That is, you would each use the camera to merely publish the video, on the web (with a password). So, you would then chat with her on the instant messaging client while accessing her published webcam separately, and vice versa. Clumsy, but it might work.
She would install software to publish the camera's view (hopefully that wouldn't be prevented by running the iChat). On your end, you'd get a wireless camera with it's own server, like this one:
http://www.trendnet.com/products/TV-IP400W.htm.
Then, since I assume you're on separate networks, you can have the fun of figuring out port forwarding, so the camera's view is accessible outside your network.
Or, get a Mac!
