The difference is trivial..something like a tenth of a percent of your total power output. Read this column from Lennard Zinn for some good test info (highest drag: GigaX at .41 watts).

The older square taper bottom bracket systems actually have less drag than modern external bearing cranksets, but virtually all of the resistance you feel in spinning the cranks by hand is due to bearing seal drag, which is relatively constant and very small in magnitude compared to the other forces resisting you.

For instance, some tire rolling resistance testing suggests that the difference between a very poor tire and a very good tire/tube combination might be as much as 20-30 watts at speed... this is an order of magnitude above the gains from any fancy ceramic bearing upgrade and even beats the gains from some aero improvements. It's just not as sexy as saying you have dimpled Zipps with ceramic bearings. ;-)