index scan on =, but not < ?

I have two index questions. The first is about an issue that has been
recently discussed,

and I just wanted to be sure of my understanding. Functions like count(),
max(), etc. will

use sequential scans instead of index scans because the index doesn't know
which rows

are actually visible.is this correct?

Second:

I created an index in a table with over 10 million rows.

The index is on field x, which is a double.

The following command, as I expected, results in an index scan:

=# explain select * from data where x = 0;

QUERY PLAN

-------------------------------------------------------------------------

Index Scan using data_x_ix on data (cost=0.00..78.25 rows=19 width=34)

Index Cond: (x = 0::double precision)

(2 rows)

But this command, in which the only difference if > instead of =, is a
sequential scan.

=# explain select * from data where x > 0;

QUERY PLAN

------------------------------------------------------------------

Seq Scan on data (cost=0.00..1722605.20 rows=62350411 width=34)

Filter: (x > 0::double precision)

(2 rows)

Why is this?

(This is with pg 8.0.1 on a PC running FC3 with 1GB ram.if it matters)