Linux router

I am having 512 kbps dsl connection to our Lan. Right i have access to clients by giving dsl router ip address as gateway and dns address in tcp/ip properties. Now i want to put my linux box in between, so that i can configure firewall, to restrict.

Kindly give some suggestion how i will implement this project. Should i have to add one more ethernet card to linux box and terminate dsl router lan output my linux box. In all clients i should give the second ethernet card ip which is connected to dsl.

I am confused, kindly give some suggestion.

Regards
Sakthi