Rebuilding Index - sort in temp db What is the significance of SORT_IN_TEMPDB when working with indexes on a table of significant size (20GB plus)?

Where else does sorting take place?
It depends.

The options are -


Sort in the destination file group (normally PRIMARY on the DB the index is on)
Sort in TempDB


If your TempDB is on a different disk array and not active, you could see a performance increase.  There are also space considerations.

There's a pretty good article on MSDN about this topic here.