limiting bandwidth on a per user/worksta

we are planning to get a 100KB-150KB /sec WAN connection for our company, either DSL or leased line. either way the connection will be shared by 20 users. is there a way to limit the bandwidth on a per computer basis, say no more than 5-6K per terminal.

either through linux setup as a proxy server or a third party product for NT.preferably freeware products if possible

please reply with website links etc.
thanks