linux pc sharing its internet connection

[Tenaka]

sorry if thi question might sound slightly stupid but I am very uncreative atm

what I want to do for some friends is this:

I want to put up an old pc connected to internet with on ethernet card and with another ethernet card t oa switch.
the pc is supplying IP addresses via dhcp and is used as a gateway.

what is the easiest and/or most elegant way to measure traffic for each user and setup certain rules? I know I could do that with a router but I need to do it with this pc.

I thought about using a proxy tool on the pc so users have to supply a password? so things do not depend on IP addresses beacuse some users do not use their pcs for weeks so another user could take the address of another user and use his bandwidth,

sorry for the confusion, hope you udnerstood what I want to do...

[till]

Have you thought about using a specified linux distribution like IPCop?

[Tenaka]

I have been looking into fli4l www.fli4l.de - also thats minimalistic... I'll look into IPCOP too

well but is there a licecd available of IPCOP for testing?
It this only a firewall? I did not find anything about traffic regulations etc. in the faq...