The Chicago Manual of Style (15th ed.), 8.13, says that in English usage  van 
and the like "are lowercased when full names are given but usually  
capitalized when only the last name is used"--e.g. Joannes van Keulen, Van  Keulen.  I 
would follow this practice for people living in Holland or even  New 
Netherland, but I believe that the general practice is to capitalize the Van  for names 
that have become Americanized.  I think Martin Van Buren is  almost always 
used.
 
David Allen
La Mesa, CA