Hi,
It is very simple to configure your linux box to act as a router. here the a steps you need to follow.
Configuring the router in linxu is called as ip forwarding. so you need to tell the linux box to forward the traffic from one network interface to other interface, this process is called as ip forwarding.
follow the following steps:
1. open the /etc/sysctrl.sysctl.conf file and put 1 in the net.ipv4.ip_forward=1 <--------- instead of 0.
2. reboot the system.
or if you want immediately avai the option,. you need to do some tweeking. it is like
open the command prompt
1) type echo 1 > /proc/sys/net/ipv4/ip_forward
that is all about to convert the your linux box into router. but you need think so many options about router like arp proxy enable on all the interfaces of router or not etc.,
if you want to more details about this go to www.routerproject.com website.
Cheers