Newbie security question

I have 3 workstations at home, 2 running Windows XP Pro and one recently
set up with Redhat 9. All machines are on an ethernet network behind a
Linksys BEFSX41 router/firewall attached to my ISP's DLS modem. I also
run Zone Alarm software firewalls on the XP machines, and the default
Linux firewall on the Redhat machine (I just chose the "medium" firewall
setting when prompted during the Redhat installation).

One of the XP machines is temporarily hosting my personal home website
and a mail server for my family. I want to set up the Linux machine for
my mail and web server.

I understand how to configure Apache and Qmail for optimal security, but
I don't understand the best way to set up linux machine itself on the
network.....is it o.k. to leave it on my local home network or should I
move it to the DMZ port of my router? Could you point me in the right
direction as to what else I need to do to have a secure network with
these 3 machines?

TIA,
- Dave Kistner