Bandwidth requirements for Audio Video Chat

How do I calculate the bandwidth required for A-V Chat?
Does Live Messenger use compression? If so, which codecs are supported?
Can I supply and specify a codec? Can I use hardware compression?

I have seen figures for Audio requiring 32Kbps and Video requiring 120Kbps
but quality and/or pixels/frame rate were not specified.

Thanks for any suggestions
....S...