Hi Davimint
Bear in mind I can't guarantee that you can get the Linksys running as a WAP/hub (haven't been there, didn't get the T-Shirt!!).
Don't worry about the WAN port (in this case it is the internet) you are only interested in the Ethernet ports.
I don't have access to a Linksys wrt54gs but found this guide on the net:
http://register.wireless.utoronto.ca/?page=linksys hope it matches your router. (BTW - you have to press the reset button for about 30secs on a Netgear with the power on to reset to default, don't know about the Linksys).
From what I can see the following are the important settings:
Disable any ISP logon stuff (username, password etc - couldn't find it in this guide).
In step 4 enter a rubbish
STATIC IP address eg: 192.168.49.49
In step 5 set the local IP address to 192.168.2.254 and
DISABLE the DHCP server
Save the settings.
Plug the router into the IPCop via an ethernet port, restart your test PC and hopefully you should be able to ping the outside world - yahoo.com. If you get this far you will now need to play with the IPCop Firewall > DMZ Pinholes submenu to start pinging across the BLUE/GREEN subnets.
I don't know if you will be using the WAP but the following should make initial setup easier:
In step 8 disable the Wireless MAC filter if it isn't disabled by default.