Questions about firewall

[fgb () domain com br]

I'm a begginer in firewall technologies, and I have several questions, so I hope the wizard 
will be able to help me a lot. ;-)

I'm using Linux Red Hat 5.2 (Kernel 2.0.36) with two NICs, one in the Internet (connect to an ISP connection throw a 
router) and another in the protect network. I have a little range of valid address and I'll have a mail and a web 
server. My first question is: Do I need to have a third NIC in my firewall machine and a little network (DMZ) where I 
will connect my mail and web server, or can I perform a NAT on the linux machine and make my servers, that are in the 
protect network, visible on the Internet ? In case of the second option, how can I implement the NAT ?

Since I'll be using Red Hat 5.2 (kernel 2.0.36), I should use ipfwadm, is that correct ?

Can I  have IP filters so that I can control access of certain protocols and ports ?

I also want to use a proxy/cache server. Is squid a good choice ?

For these caracteristics I pretend to have in my firewall, what services may I compile in the kernel and what modules 
should I install ?

Any ideas, tips, pointer, etc, would be much appreciated.

Thanks,

Fabio.
fgb () domain com br