trouble sharing my internet connection
 

Hello. I'm not an actual newbie to Linux, but I stumbled upon a problem that I cannot find an answer for.
I installed Ubuntu 8.04 Server to an old PC in order to make a small router and torrent box out of it. I use Webmin and PuTTy in order to manage the server. I connect to the internet using PPPoE over eth0, and eth1 connects to a switch where my 2 desktops (mine and my parents') are also plugged in. I have DHCP server enabled for the local network and I've also set up NAT so that I can have internet on the 2 desktops.
The problem is that I cannot make the 2 desktops access internet simultaneously. Whenever one is used to surf the web (just surfing and Yahoo! Messenger, not necessarily heavy downloads or torrents) the other one cannot open anything related to internet (browser, messenger, antivirus updates, etc). So they can only take turns in using the internet, which is very annoying, as you can guess.
My questions are why is this happening and what can I do to make things work as expected.
Thanks in advance for your answers.

Hmm...not sure of the details of how you have things configured, but try to install Squid, which would easily let multiple systems share a single internet connection.

But that may be overkill. You could always configure that box to forward any DNS requests to your ISP's name servers (just edit /etc/resolv.conf to point to whatever your ISP tells you), and set a route for your internal network to your internal NIC, and route everything else out your ISP interface.