How to get higher tps

I'm exhausted to try all performance tuning ideas, like following
parameters

shared_buffers
fsync
max_fsm_pages
max_connections
shared_buffers
work_mem
max_fsm_pages
effective_cache_size
random_page_cost

I believe all above have right size and values, but I just can not get
higher tps more than 300 testd by pgbench

Here is our hardware


Dual Intel Xeon 2.8GHz
6GB RAM
Linux 2.4 kernel
RedHat Enterprise Linux AS 3
200GB for PGDATA on 3Par, ext3
50GB for WAL on 3Par, ext3

With PostgreSql 8.1.4

We don't have i/o bottle neck.

Whatelse I can try to better tps? Someone told me I can should get tps
over 1500, it is hard to believe.

Thanks

Marty