FK index q'n

Given this table:

CREATE TABLE foo (
     id integer primary key,
     bar_id integer references bar (id)
);
and provided that bar.id is itself a PK, do I still need to create an
index on bar_id if often doing queries like:
SELECT MIN(id) FROM foo WHERE bar_id IS NULL;

Table foo will contain a static number of rows (from 2,000 to 10,000 --
yet undecided) only doing SELECT & UPDATE.

Thanks.