Where is my bottleneck?

Hi all,

I have a performance problem and I don't know where is my bottleneck.
I have postgresql 7.4.2 running on a debian server with kernel
2.4.26-1-686-smp with two Xeon(TM) at 2.80GHz and 4GB of RAM and a RAID
5 made with SCSI disks. Maybe its not the latest hardware but I think
it's not that bad.

My problem is that the general performance is not good enough and I
don't know where is the bottleneck. It could be because the queries are
not optimized as they should be, but I also think it can be a postgresql
configuration problem or hardware problem (HDs not beeing fast enough,
not enough RAM, ... )

The configuration of postgresql is the default, I tried to tune the
postgresql.conf and the results where disappointing, so I left again the
default values.

When I do top I get:
top - 19:10:24 up 452 days, 15:48, 4 users, load average: 6.31, 6.27, 6.52
Tasks: 91 total, 8 running, 83 sleeping, 0 stopped, 0 zombie
Cpu(s): 24.8% user, 15.4% system, 0.0% nice, 59.9% idle
Mem: 3748956k total, 3629252k used, 119704k free, 57604k buffers
Swap: 2097136k total, 14188k used, 2082948k free, 3303620k cached

Most of the time the idle value is even higher than 60%.

I know it's a problem with a very big scope, but could you give me a
hint about where I should look to?

Thank you very much
--
Arnau