Bandwidth management

I have a windows 2000 server and 12 clients connected to me in a lan.clients have different op systems like win98, win95, linux, mac etc. I am using sygate access server to provide them internet.I have 128Kbps connection on my server.I wants to limit internet speed on my clients.For example i wish all my clients to use 12Kbps and not to exceed this limit.So any one can tell me wich software to use or is there any other idea to control speed of internet on my clients.