DMZ Design and Functionality

[Dana Rawson <absolutezero273c () nzoomail com>]

Forgive me if these questions are too basic but I am relatively new to 
this.  I am the network administrator at my company and over the past year 
have become aware of a need for increased security.  I have been reading 
posts here in hopes of learning more about this.  While I have learned 
considerable amounts, and have searched for answers elsewhere, I am still 
in need of guidance.  Any help or direction would be greatly appreciated.  
I am open to reading any books that one might recommend.  I have seen a 
few books out there but not sure which are worthwhile.  

Anyway, my background information is this:
I wanted to install a DMZ at 2 of my company's locations.  I do have a 
limited budget so I was planning on using OpenBSD for my first tier 
firewall.  I do have a hardware based firewall in place currently which I 
was planning on using as my second tier firewall.  
My initial plan is to build a machine using OpenBSD that does nothing but 
firewall.  Additionally, I wanted to add another machine to run 
Sendmail/SpamAssassin and an an anti-virus software.  On this I was hoping 
to run Redhat as this is what I am most knowledgeable on.  My thought 
behind this was to block spam, of course, and also run a gateway anti-
virus solution that would block viruses coming from websites and 
employee's personal e-mail accounts.  This due to the fact that I have 
seen a number of viruses coming in from either their 'webmail' or through 
their Outlook Express. I wish to set up an ftp server and webserver to 
facilitate OWA.  Additionally I would like to make available VPNs and 
encrypt all data transmitted over remote connections.  Remote connections 
may consist of a MS RAS and Citrix.

With this information my questions are:

1. To begin, does this sound like an acceptable solution?
2. How do I size the machine that I am going to run OpenBSD?  I have read 
that a DMZ will slow performance down some.  If I have a fast enough 
machine will it aid performance?  At what point is overkill when running 
OpenBSD.
3. How do I size the machine that will be running Redhat, Sendmail and 
SpamAssassin?  Is this configuration acceptable?  Should the Anti-virus 
software be running on a separate machine?
4. What open source options to I have for encryption and VPNs?
5. Are there any potential problems running this configuration?  Does 
everything mentioned here play nice together?  Would you change anything 
here and if so why?

Many thanks in advance.

Dana

---------------------------------------------------------------------------
----------------------------------------------------------------------------