WoW is surprisingly light on bandwidth usage. There are some players that have pushed upwards of 25 clients running on a standard DSL service line with little noticeable lag. And keep in mind, I believe there are still WoW players using dial-up connections.

If you'd really like to measure out what WoW is using try a bandwidth monitoring program like : Bandwidth Monitor
run that little program during a session and you can see your upload/download speed. Also, in your task manager under the Networking tab you can get some date that should give you a feel for your utilization.

I run 5 clients on some of the worst cable internet service available and average 140-200ms lag times. Which is doable.