You are an ISP?! In that case, I would HIGHLY suggest that you contact a specialist in networking that works with ISP's. I am not dis-crediting the help that is offered up on the forums, but a SOHO environment if MUCH different than an ISP.
You will likely be looking for a Cisco-like device, with which you can perform traffic shaping in the hardware. Depending on the number of customers, and the way in which the infrastructure is set up, I HIGHLY DOUBT that a software solution will be possible. Extreme Networks hardware is pretty good from what I have been told by peers at work that deal with Customer Equipment, they come recommeded OVER Cisco.
Extreme Networks