since i cannot think strate at the moment i will give you the most simplest answer i can posibly think of.
It is called Mandrake multi network firewall. it has funky stuff like this built in.
if you beleive you are to saperia to use this really good tool, then the next best option is to use a squid
to proxy all requests. Then you can use squids built in throttling techniques.
and if you are into some hard core hacking you can play with the linux routing table to create a hard limit on data coming from each ip address. Though there is a tool which can do it for you, called CBQ
ftp://ftp.equinox.gu.net/pub/linux/cbq/
hope i helped
