What's a good ND filter to use with fast lenses in bright daylight?

[powervideo]

I have a couple of pola's on order but what would type of ND filter would bring the aperature down to f2.8 or f/4 at 100 ISO / 50th sec on a bright sunny day? Would a ND6 or ND8 do it? I'm not sure about the Fasder ND and want to look at regular vanilla ND's.

Thanks,
Peter