Index before or after bulk load using load infile?

I have a database with over 1B rows and two columns that are indexed (in addition to the PK). Is it better to have the index pre-defined in the table before the load infile or better to index after the data has been loaded? A couple of notes regarding data size and system: - System is Linux w/ 8 cores and 32GB memory (currently maxed out unless I move to new HW) - DB is 1B rows that in raw data size is 150GB data. - Database is MyISAM and is mainly read-only after it's loaded.