Route traffic through outside proxy

Hi
here is what i want to do.
I have a 750GL router board
(e.g.)
Port 1 is connected to Internet
Port 2 is connected to Internal network
right now users that are connected to Port 2 are masquareded to get internet
i want to put an external proxy (squid) between them so the users ip is the outside proxy server ip..
any idea?

This is not an easy thing . it depends on your bandwidth , but rb750gl cant held all that.