webserver behind firewall

I am looking to set up a web server on my home machine that is on a t-1. I
have looked at a number of docs on port forwarding, but, I still don't
understand it.
Heres the situation...
I live in an apartment complex in a small town. We go through a local
provider who contracts out to a larger isp. I have a rh machine set up as
my gateway, fine.

heres how it looks:
Main ISP - blah.net (206.xxx.xxx.xxx) a server at my complex
(192.168.xxx.xxx) my outside nic on my gateway: (192.168.xxx.xxx) my
inside nic (static 192.168.xxx.xxx) I want to run the server on an xp
machine with a static ip (192.168.xxx.xxx) webserver answers on port 81

lots of layers.
Heres what I've tried-
iptables -port forward from the main ip (206.xxx.xxx.xxx) to my windows
machine at port 81 ... no go.

I am confused (to say the least!)
questions:
in the end what ip address will my users use? the main one?(206......)
how does the flow to my web server supposed to go?
can someone point me in the right direction? I don't necessarily want a
step by step, I just want to understand how the requests get
through.............

thanks,
patrick