Amahi supports one ethernet connection and has no options for NAT that I'm aware of.
If Amahi has no support for NAT I see no use of DNS and DHCP, those 2 services are best to be used on the same system that provides the home-networks computers NAT.
Amahi's DHCP and DNS servers are mainly a means of making its installed applications easily available to other machines on the LAN. It is (currently at least) aimed mainly at home networks, most of which will have a DHCP server built into a router, but few of which will have anything like a DNS server. In its design role, it has no need for NAT or routing services, and, so far as I am aware, has no provision for them at all. Since it is Linux, they could be added, but I think it is questionable whether that is worth doing or not. Having a local DNS service is useful to a home network, and (in my expericne at least), the Amahi DHCP server is quite a bit quicker at IP allocation than the DHCP server in my Netgear router.
Amahi also currently supports only one ethernet port. As Moredruid has outlined, it is possible to add another port, but you would have to use that as your WAN interface, and leave eth0 for internal services. You would also need to add a firewall, since Amahi, by design, has no need of one, and does not have one.
Thanks for the advice on SME and eBox, I will try those two options.
Both are easy to set up as gateway servers, and can easily provide routing and NAT services. Neither have the application infrastructure that makes Amahi so appealing. eBox is the more flexible of the two, but also the more complext to set up. eBox is especially useful for people with more than one WAN interface, since it can provide load balancing and failover services for multiple WAN interfaces.
Using either with Amahi may be complicated, since both also possess DHCP and DNS servers. I have one customer that I have set up with an SME server as a gateway/mail server and an Amahi server as an internal application/file server. This is working fairly well, but I did have to add hostname entries for every Amahi application that he has installed (since I am having to use the SME DHCP and DNS servers). For some reaons, even with a hostname entry, Feng office will not work on one machine in his network.
Since SME is providing gateway/routing services for his network, I don't think I can afford to turn off its DHCP/DNS services (

I'm not even quite sure that I
can turn them off), but it would be very useful to be albe to use one as a DNS relay, so that Amahi applications could be used more directly.
Paul.