After more testing...

The other machines on 172.20.1.1 had the wrong netmask (I've changed them to 255.255.255.0 now, which gets rid of the host is down message).
The other machines have been set to use either 172.20.1.1 or 172.20.2.5 as the default gateway, depending on which network they are on...

From the Win 9X box (172.20.1.107)

Active Routes:

Network Destination        Netmask          Gateway       Interface

          0.0.0.0          0.0.0.0       172.20.1.1    172.20.1.107

        127.0.0.0        255.0.0.0        127.0.0.1       127.0.0.1

       172.20.1.0    255.255.255.0     172.20.1.107    172.20.1.107

     172.20.1.107  255.255.255.255        127.0.0.1       127.0.0.1

   172.20.255.255  255.255.255.255     172.20.1.107    172.20.1.107

        224.0.0.0        224.0.0.0     172.20.1.107    172.20.1.107

  255.255.255.255  255.255.255.255     172.20.1.107               2

Default Gateway:        172.20.1.1

===================================================================

From the MacOSX box

default 172.20.2.5 UGSc

To recap
Any machine on 172.20.1.0 (original network) and 172.20.2.0 (new network) can ping the NAT box and access the internet
The NAT box can ping any machine.
The sparc5 box can ping any machine (because it has ip addresses on both networks it won't need to route it's own ping traffic)
A normal machine on the 172.20.1.0 network cannot ping a normal machine on the 172.20.2.0 network (the reverse is also true).

A traceroute from the MacosX box (172.20.2.47) to another machine (172.20.1.105) gives

1 172.20.2.5 (172.20.2.5)

2 * * *

A traceroute from the Acorn box (172.20.1.105) to the MacosX box gives

1 172.20.1.1 (172.20.1.1)

2 * * *