Supposedly, RDP uses about 24k.
VNC is a variable bandwidth protocol.
There are many user-configurable variables like resolution, color, screen refresh, etc.
I know it works on a decent dial-up modem connection.
Given all that, I doubt you'll be able to find any hard numbers.
In your situation, the problem is your DSL uplink speed. It could be as low as 128k.
More than 2-3 people coming in through a basic DSL line and you're likely to run into problems.
Unfortunately, the only fix is to up your DSL bandwidth (if possible) and paying out more for a 'business-class' connection.
