http://linux-ip.net/html/nat-statel
http://linuxreviews.org/dictionary/NAT/
Now with iptables NAT i use iproute for routing. Its easy to configure.
This helped me a lot:
http://linux-ip.net/html/adv-multi-inte
Here you can learn some more(for best understand before doing):
http://lartc.org/howto/lartc.rpdb.multi
I have done this too(but may not help you though just for reading:
http://www.ssi.bg/~ja/nano.txt
Don't miss to read the links inside the above links.
This one has a lot of help about security. Once surf this, you will go mad(like me) worrying about security(once done everything you will feel like heaven):
http://www.linuxquestions.org/quest
I will add more as & when i find them
HTH some one