dir_index is a hashed b-tree implementation for ext3, it's riskfree (Fedora has shipped several releases defaulting to it without incident), and adds a bit of performance to your filesystem.
to update an existing partitions:
0) open a terminal or enter cli
1) sudo tune2fs -O dir_index /dev/hdXY (where X indicates device, normally a and Y indicates partition, normally 1)
2a) sudo updatedb
alternatively, unmount and do
2b) sudo e2fsck -D /dev/hdXY
Wasn't that easy?