PG 8.3 and server load

I'm on a CentOS 5 OS 64 bit, latest kernel and all of that.
PG version is 8.3.7, compiled as 64bit.
The memory is 8GB.
It's a 2 x Dual Core Intel 5310.
Hard disks are Raid 1, SCSI 15 rpm.

The server is running just one website. So there's Apache 2.2.11,
MySQL (for some small tasks, almost negligible).

And then there's PG, which in the "top" command shows up as the main beast.

My server load is going to 64, 63, 65, and so on.

Where should I start debugging? What should I see? TOP command does
not yield anything meaningful. I mean, even if it shows that postgres
user for "postmaster" and nobody user for "httpd" (apache) are the
main resource hogs, what should I start with in terms of debugging?