Well, 128-bit isn't hard to break, and 64-bit is a complete joke. I would use 128-bit as the best option (and, in fact, that is what I use).
The best security you could get right now with TiVo is:
* 128-bit WEP
* Turn off SSID broadcast
* Use MAC filtering on the router to limit the clients allowed to connect
This isn't perfect, and anyone who really wanted into your network could still get in - there are tools that spot networks even with SSID broadcast off, MAC can be spoofed, and WEP can be cracked. But it keeps casual folks, neighbors, etc, from using your network.
I do hope TiVo implements WPA2/802.11i at some point. Depsite some statements in threads here, WPA2/802.11i is *not* weak, and there are no successful cracks at this point. Just brute force attacks on network using short/weak passwords, and that's a human issue.