I was browsing the web and stumbled upon this really cool program.
http://www.skype.com
It's made by the people who made kazaa.
I've tested it with people who's access speed ranges from Cell phone internet (144K) to people with a 6MB net connection.
All chat sessions had no noticable lag.
Great if you are in car and want to voice chat with friends.
It's free.
Give it a shot.
Peace,
Rafster