sequence scan on PK

Hi,

I understand that when a table contains only a few rows it is better to
do a sequence scan than an index scan. But is this also for a table with
99 records?

A table contains
id integer (primary key)
name varchar(70)
parent integer
comment text
owner integer
inheritAccess integer
defaultAccess integer
sequence bigint
contentsinheritaccessmove integer
contentsinheritaccessadd integer

explain select * from tblFolders where id=90;
QUERY PLAN
-----------------------------------------------------------
Seq Scan on tblfolders (cost=0.00..3.24 rows=1 width=50)
Filter: (id = 90)

(I have analyze table bit still a sequence scan).

With how manys rows it is ok to do an index scan or sequence scan? How
is this calculated in pg?

Regards
Jer