Justin.tv is a streaming solution we have just started using. It is easy to setup and customize and can be used from Windows and Mac OS X. It includes a chat feature but each participant would have to have a justin.tv account to be able to chat, so not a real convenient web conferencing setup with respect to integrated chat. But for just streaming the audio/video it works well and is very simple to customize. I used the video plug-in code to make a customized page, including links for different video sizes.
We purchased a high def Canon XH-A1 camera, so that should be very beneficial for streaming to be able to zoom in with high quality to view the presentation and different people as they talk.