I was thinking of replacing mine as preventive maintainice. I can't figure out what part it is on realoem.com

do you mean the fan that's attached to the water pump...(#2) in the diagram? Leaving a link to the diagrams you were looking at would have been helpful and pointed us to what may have been confusing you.

http://www.realoem.com/bmw/showparts.do?model=GK23&mospid=47533&btnr=11_1534&hg=11&fg=35

Yup that is the one. Do you change both the coupling and the pulley?