DMZ or not ?

[fgb () domain com br]

Hello wizards,

Divergences are occurring here im my officce about the use of a DMZ, and I hope the wizards will give me some 
explanations and/or secure informations about the better 
implementation.

Currently, we're using Linux as a Firewall Box, with a port forwarding to our mail server, that is behind the firewall.

We are in way now, to install a public web server and a DNS server. What are de advantages and disadvantages of placing 
this servers behind the firewall and perform 
NAT or Port forwarding, instead of  using a DMZ ? 

Which of the options shoud I implement here in my officce, to have a secure site ?

Thanks and regards,

Fábio Baptista
fgb () domain com br