Port Forwarding: 8080 to 80 from WAN, 80 from LAN

Hi there, first time in here.
I have XAMPP installed on mi Ubuntu 9.04, with Apache listenning at port 80 (default) and I want some friends to see my webs through the Internet. The simpliest answer is: tell them to visit http://yourip/ and that's it.

The problem is that i'm behind a Zyxel Prestige 660R-61C router, so I have to do port forwarding.

In it's NAT section (under "SUA Only"), the router let's me set a port range to be redirected to a determined IP. So, telling it to forward port 80 to my LAN IP would be enough, except that port 80 at the router is being used to display the web interface.

So, my new idea: using port 8080 from the wan. I set the router to forward port 8080 to my lan address, but I don't want Apache to listen at port 8080 (don't want to use :8080 locally).

So, how can I locally forward my incoming 8080 requests to my 80 port?

I've never used iptables, I tried some command I found on the net ( "sudo iptables -t nat -I PREROUTING -p tcp --dport 8080 -j REDIRECT --to-port 80" ), but not worked - or, at least, it don't seems to work.

I've installed Firestarter, and set some rules:
Code:
At INCOMING:
Allow service  |  Port  |  User
HTTP  | 80  | everyone
HTTP-alt  |  8080  | everyone

Forward service  |  Firewall port  |  To  |  Port
Http-alt  |  8080  |  localhost  |  80
(actually, my firestarter is in Spanish, so the translations could not be accurate at all)

y Applied changes, and it still doesn't work.

What shall I do?

Thanks for reading, in advance..

EDIT: attached a nice picture to ilustrate what I want (yeah, I have to learn how to use GIMP too )