Running A Server Behind the Router
Thanks to Bob Kunzig for this.
The following steps will enable you to run a Q3A (and probably any other game server such as Shogo) server. I tested these steps running a Shogo server and it worked.  Before you start, copy down and save the current settings (Status page) on the router for your reference.

1) Set your PC to a static, Class C address (192.168.1.100, for example).

2) Set your router to the static IP address your ISP assigned to you. (check your reference)

3) Turn off the DHCP feature of the router and use port forwarding for Port 27960 to the local
static IP of your server machine (alternatively, you can use the DMZ feature or you can use BOTH). 

4) Set the default gateway to the isp IP address for the gateway. 

5) set the DNS to point to your ISP's DNS. 

6) Run the server. 

Issues
I tried this over my cable modem and two users connected via 56k modems. They experienced exterme lag. Chances are if they had also been using a high speed connection this would not have been a problem (or as much of one). Some people doing this report a lot of lag and some don't. There is definitely going to be some delay using the router. 

Another issue was that the server did not show up in Gamespy even when entered in Unbound. The users instead entered the server ip in-game and were able to connect. 

Running Multiple PC's Behind the Router to an External Server 
Thanks to Chris Nielsen and mad_hatter_98 for this:
This is essentially classified as a many to one connection. That is, multiple concurrent copies of the same game through the router. (There is another type of router which uses statesful inspection as opposed to NAT which is what the LinkSys uses. This other model can be seen here.) In order to run Quake3 (and probably other games running concurrently) from many PC's through the router do the following: 

Put the following line in the config.cfg file for Quake in the second computer:

SET CLIENTPORT 2790x

For example set the x to a 2 (27902)

From there everything works fine. The clientport defaults to 27901, so you need to change it on every other computer on your router.

Note: One of the articles below lists the setting as NET_PORT . Try both and see what happens. 

Additional Info Links:
LinkSys Router Reviews and Help

Practically Networked Review

g256 Review

Practically Networked Router Help
 

Apparently the Zyxel P312 and Zywall10 are the same product with the P312 being phased out...