- Make sure it is a version that can be loaded with a third-party firmware. For a list of compatible serial numbers, look here. Notice that the ones starting with "CDFB" cannot be loaded with third-party firmware.
- Get Tofu's latest modified HyperWRT firmware for the WRT54G at linksysinfo.org (you must be registered to use the download section).
- Flash the new firmware using Linksys' TFTP utility (you can also use the web-based utility in the router, but the TFTP utility is safer).
- In the router's setup, go to Wireless.
- Change the Wireless Mode to "WET" (Wireless Ethernet Bridge).
- For the SSID, enter the name of the AP you'll be connecting to.
- Go to Administration, and click the "Reboot" button.
- After the router reboots, go to the Setup page.
- Set the router IP to something within your other router's subnet (such as 192.168.1.2). Your other router won't be able to assign an IP to this interface. UPDATE: According to Thibor, the developer of the firmware for the WRT54GS, wireless performance may be improved by setting this IP to something not within your other router's subnet. For example, if your main router's IP is 192.168.1.1, set your "bridge" to an IP of something such as 10.0.0.1. Once the router is set to bridge mode, there is not much need to access its setup page. If you ever need to do so, plug a computer directly into it, and set the computer to a static IP of 10.0.0.2, for example.
- Disable DHCP (devices connected to the WRT54G, such as your TiVo, will get their IPs from your main router).
- If you use wireless security (WET, WPA, etc.), set the WRT54G to match the main router's settings in the "Wireless Security" tab.
A WRT54GL is also an option, as it is basically a WRT54G version 4 (use Tofu for this router).
Wal-Mart has the WRT54G for $49.97 now, but I got mine at RadioShack for a little more (but it also had a rebate). A guy in the store actually checked out the serial numbers while I was on the phone with him (Wal-Mart only had version 5 routers).
Note that I actually had my TiVo working with an old Hawking 802.11b USB adapter, but it turned out to be way too slow (6.2 software doesn't support 802.11g adapters).
It seems that the ZyXEL Prestige P-330W can be purchased for under $50, and it supports ethernet bridge mode out of the box.