Insert performance

Dear all,

After many tests and doc reading, i finally try to get help from
you...

Here is my problem. With some heavy insert into a simple BD (one
table, no indexes) i can't get better perf than 8000 inserts/sec. I'm
testing it using a simple C software which use libpq and which use:
- Insert prepared statement (to avoid too many request parsing on the
server)
- transaction of 100000 inserts

My server which has the following config:
- 3G RAM
- Pentium D - 64 bits, 3Ghz
- database data on hardware raid 0 disks
- x_log (WAL logs) on an other single hard drive

The server only use 30% of the CPU, 10% of disk access and not much
RAM... So i'm wondering where could be the bottle neck and why i can't
get better performance ?
I really need to use inserts and i can't change it to use COPY...

Any advice is welcome. Sorry in advance for my bad understanding of
database !

Thanks in advance.

Regards,

Joël.W