Quota on the bandwidth. How can I do this ?

Hi experts,

I have a kind of mini call center that is connected to the Internet through a partial T1 line of 512Kbps. Now, we're going to share our Internet connection with others in the town, a kind of NISP. We've already ordered more bandwidth to 1024Kbps, but I need to make sure that those people just use 512kbps of our connection. I'm a kind of confused how to set this up.
My current topology is -> (Internet)<->(Cisco 1721)<->(LinksysRV082)<->(Intranet)
We would change the Linksys RV082 for a switch that we didn't have available, but I don't if will be a good choice now because of the new connections.

So, I have a Linux box that is NOT currently on the network, with two ethernet cards that will make the connection between the people and us (Internet). This box will be configured to be the DNS, e-mail, http server, Bandwidth control, etc ...

My questions are ... I don't want that they use more than 512Kbps, so how can I limit this ? Is this a setting on the router or on Linux ? Is it possible through the tc command in Linux ?

Do you have any suggetions regarding the network topology in general ? How can this be improved ?

Thanks for your help,