Router, firewall, both, confusion...

Which piece of the puzzle handles NAT / port redirection? I'm thinking
either, but my opinion is based on very limited experience with GTA
firewalls such as the old RoBox and GnatBox-Pro.

My real problem is I need a new firewall. My RoBox died and they want too
much to fix it. The GnatBox-Pro is obsolete and I need some newer features.

There is a Cisco T1 router belongs to the ISP with a single net connection
to me. That connection is going to the old GnatBox-Pro.

I have a block of 8 IP addresses from the ISP which let's me use 5 of them.
I have domains and web sites on each of the 5. All incoming SMTP and POP3
traffic to go to one internal IP address to either an exchange server or a
standalone mail server (currently on the private network). All the port 80
stuff currently goes to a different IP on a machine in the DMZ
(semi-private). The port 80's are remapped to ports 1080, 2080, 3080, etc
on a single web server box.

I am looking at either a new GB-200 or a Cisco 871 to replace the
GnatBox-Pro. The main plus for the GB-200 is I know it will do what I want.
The question I have is, is what I described generally true of
router/firewall products or is it unique to the GBA products?

Scott