To check bandwidth usage, i recommend the program
MenuMeters
It will allow you to display bandwidth usage (up and down) in the menubar. It works in 10.3 and 10.4 with no problems. I set the colors to black and it looks polished enough to have been included in the OS.
I've yet to test tiger to tiger vid chatting so I can't tell you how the new iChat compares in bandwidth usage. iChat scales video resolution based upon the processor speed as seen in the system profiler... so usage will depend on the speed of the computers involved.