WPA is much better than WEP as the keys used to encode the data change with WPA, unlike WEP. WEP keys can be cracked quite easily by anyone which a PC.
If you have an TiVo Airnet card, then you can only use WEP - they don't support WPA.
If you use an external wireless card, which connects to the TiVo via Ethernet (as suggested in post#2) then you can run whatever you feel like!!
Ethernet (n): Something used to catch the Etherbunny
Tivo 1: 6023, Cachecard & 500GB (WD)
Tivo 2: VirginMedia HD