Personally, I don't think using the one machine as a router is going to be particularly safer than doing it simply using a router/switch/hub/whatever that you're using now. Some will say that iptables makes a much stronger firewall than using a store-bought router, and they may be right... but the pain and heartache of setting up iptables yourself is almost not worth the security its buying you.
In my case, I have the router splitting the connection to each of my boxes and using Zonealarm as my firewall on each of the machines.
If you setup all of your machines to be connected to the router (normal hardware router... not a PC) you can do port forwarding from the router to the system you plan to use as the mail server. That way, all your mail can still reach the box. Then, you can setup your mail clients on each of the PCs in your house to retrieve mail from the mail server.
This isn't too hard, but the thing you have to be aware of.... if you install a mail server on a system that has a dynamic IP address, some ISPs on the internet may block your server. I have heard tales that some ISPs will block any IPs that are dynamic, preferring only static IPs. Anytime you run a server, it's best to have it on a static IP... otherwise it makes domain name changes a little bit difficult.
For example... you have your email account is firstname.lastname@example.org
. YOu would have to setup your server so it answers for yourdomain.com and can receive mail for this domain too. That means when your IP changes (since your IP is dynamic) the domain now points to the wrong IP! There are ways around this using services like zoneedit.com but they can also be tricky with email servers who prefer you have proper reverse DNS setup.
Setting up a server is no small affair.