Install configuration questions

[Brian Bucher <babucher_(_at_)_alum_(_dot_)_mit_(_dot_)_edu>]

Greetings,

I'd like to ask for some advice on my proposed network setup.
What I would eventually like to have is this:

Internet -> Router -> Webserver (OpenBSD running IPFilter; webservice)
               |
           Firewall (OpenBSD running IPNAT and IPFilter)
               |
           Internal Server (OpenBSD running email/fileservice)

1. Is this an ok setup?  From the documentation I've read, this seems
   to be the best setup.

2. Where should I run DNS?  It seems to me I need to run it on the
   firewall machine, and that it shouldn't affect security too much.

3. I'd like to run the email service on the internal network server, so
   I'll somehow need to have the firewall forward those packets.  Since
   the internal server will be NAT'd, how do I tell the firewall to
   forward the emails to the internal server?

Any pointers to documentation would be appreciated.  I'm reading the
OpenBSD FAQ and IPFilter FAQ, but haven't gone all the way through yet.

Thanks for any advice!
Brian