Optimizing DB Permormanance

Hi there,
I am having a postgres DB on Linux Server.
The size of DB is near abt 1.5GB and also my site is on the same
server. I am having problem that my site is very slow.
I checked size on my HDD using df -h command its showing me only 15%
disk space is allocated. But when I saw the memory allcated by the
processes using top command it shows me that CPU % is reaching near
abt 90 - 90% for postmaster.

In my PHP code there is lots of SELECT stmts also while generating
report insert and updates stmts are also there. BUT on my local PC it
works fine for fewer records. while on my production server records
are near abt 5211 it is very very slow. Sometimes I get Page not found
error.

1. So can anybody tell me what will be the reason postmaster is taking
lot of CPU usage??

2. If I created CLUSTERed Index is it feasible ? Can it improve my
speed of Retriving records ?

Or if there is any other solution please let me know...

Thank you in advance
Regards
Pandurang

pandurang(at)manas-solutions(dot)com