I hope this helps somebody. This will effectively allow you to add the 4 wired ports on the WRT54G to your existing router. It should work on Linksys WRT54G v1 - v4 routers. What I used/did is in parentheses.
1. Download DD-WRT from
http://www.dd-wrt.com/. (I got v2.3 Mini)
2. Flash your Linksys WRT54G with a connected PC (wired). (I used the web interface)
3. If you have Wireless MAC Filtering enabled on your main router, then you need to add the MAC address for the Wireless to your list of allowed clients. (Get this from Wireless->Status, MAC Address)
4. Disable the Firewall. (Security->Firewall, Disable)
5. Change the Wireless Mode. (Wireless->Basic Settings, Wireless Mode=Client Bridged, Wireless Network Mode=Same as your Main Router, Wireless Network Name (SSID)=Same as your Main Router)
6. If you have security on your Main Router, set it the same. (Wireless->Wireless Security NOTE: I have WEP, so I know that works. I'm unsure about anything besides that)
7. Set the Local IP Address and Subnet Mask. (Setup->Basic Setup, Local IP Address=Unused IP in the same net block. For example, my Main Router is at 192.168.7.1 and is set up to use DHCP starting at 192.168.7.100. I set the WRT54G to 192.168.7.2. Subnet Mask=Same as Main Router. Mine is 255.255.255.0)
That should do it. Just set up whatever you hook into the WRT54G to use DHCP and it should obtain an IP from your Main Router. Also, one thing that may seem odd is it will say your WAN IP is 0.0.0.0.