Indexes on Large Tables

I am running a dual athlon 1800 with an gig of ram.  I am running postgres
7.2.  I have a table that hs 15 million rows.  I have a query that has one
columns and the select cluase and one column in the where clause with no
joining or sub-queries.  If I do not index the column from the where clause
the query returns the 150,000 rows in 4 and a half minutes.  If in do index
the column from the where clause using a btree the 150,000 rows return in 11
and a half minutes.

Any insight would be greatly appreciated.

Donny L. Drummonds