I have seen no mesurable difference in performance from 64 bit to 256 bit WEP. Personally, I'd stick with the highest number your equipment supports. You'd be surprised about who's driving around or listening in from the neighborhood. I had a friend over and we were cruising around our neighborhood, it was amazing the number of unsecured wireless networks there were! We found 16 sites, and only two of them (mine was one) had WEP enabled.