Storage Architecture!

Hi,
I am re-writting an application for my company, which uses the volume of
1 million to 2 million records per day.  Currently the application is
exisiting in ctrack and partially in Visual Foxpro.
I suggested my company about using PostgreSQL and went to a win to win
situation.  After research with postgresql 7.2.1 on windows with ultra,
(i suffered a lot to compile 7.4.1 for windows)  i switched to linux 8.2
with postgresql 7.4.1.  The performance is comparable with MySql and
MsAccess when i run Postgresql on Linux.  Its lagging only in the select
query.
Well anyways, my problems is to get to know about the storage
architecture of the database for where he is pinching on me.
Regards,
Mohan