One resource on Spanish names of Germanic origin (
http://www.flick.com/onomastikon/Europe-Western/Spain/Germanic.htm) notes that Arnulfo is derived from
Arnold.
According to the BtN database, Arnulfo was the 843d most popular name in the US in 1990 (0.0069% of the US male population). I am afrais it did not make the top 1000 in 2000.