Which is better (more columns or rows) ?

Hi,
  I have recently tried some simple test on the postgresql 7.0 running
on a PIII 600MHz, 128 Mbytes RAM . I created a simple Visual Basic app
that query two tables.
The first one consist of 10 columns  and the other 30 columns. I
inserted about 250,000 records into each tables and then do a "vacuum"
on the database.
Next I query to select about 100,000 records. I repeated this query for
5 times and each time I will do a "vacuum".
Below is the average timing I get:-
For 10 columns table ~109s
For 30 columns table ~ 112s

Is it true that I can design database tables to have more columns
without performance degradation during query ?

Regards,
Thomas Wong