Using linux as a router

I have a Fedora Core 6 basic install (no GUI or anything). I have the NICs set up between two networks (one on each) and I can ping hosts in each network and those hosts can ping it. My Question is: how do I set it up to route between the two networks? I have spent so much time trying to learn iptables, but can’t seem to figure it out. I did the enabling of the routing by “echo 1 > /proc/sys/net/ip_forward” thing and tried adding “FORWARD_IPV4=true” to the /etc/network file. I understood that the rest is done through iptables, but I can’t seem to figure it out. I don’t want to NAT, I just want it to route. Help me please.