Just a simple(?) routing question for the Linux experts out there...

Background
I have a Linux box which connects to the Internet and does NAT based internet sharing for the rest of the network. This box also has a dial in modem attached, and I can dial up with my laptop (or phone) to gain access to my home network and the internet.

I am moving house in the next few days, but my always on connection cannot be moved (the local telco won't install new ISDN lines, and ADSL is a distant speck on the horizon... =( I want to move all my computers to the new place, except for the NAT box.

I have a second linux box for the new place, and it can happily dial into the NAT box, and gain access to the internet, however any machines connected to it cannot.

So the question is, how do I get the new linux box to pass packets to the NAT box?

Some information
NAT box is on 172.20.1.1, The new box is 172.20.2.5, all the machines moving house will have their network addresses changed to the 172.20.2 range)