Ok on the NAT box...

Dest        Gateway            Genmask        Flags IFace

172.20.2.0  sparc5(172.20.1.5) 255.255.255.0  UG    eth0

172.20.1.0  *                  255.255.255.0  U     eth0

The other routes on the NAT box are for my Inet connection (including the default route out which is on a 212.x.x.x address)

On the New box (sparc5, 172.20.1.5 on eth1 and 172.20.2.5 on eth0)

Dest        Gateway     Genmask        Flags  IFace

172.20.2.0  *           255.255.255.0  U      eth0

172.20.1.0  *           255.255.255.0  U      eth1

default     172.20.1.1  0.0.0.0        UG     eth1

I've also ignored the loopback routes on both machines.

As I said, the default route works for Inet and the NAT box itself, but doesn't work for other machines on the 172.20.1.0 network. Adding a gw for the 172.20.1.0 network on sparc5 stops the 172.20.1.5 address from responding to pings from the 172.20.1.0 side