I concur with Merlin you're I/O bound.
Adding to his post, what RAID controller are you running, does it have
cache, does the cache have battery backup, is the cache set to write
back or write through?
Also, what do you get for this (need contrib module pgbench installed)
pgbench -i -s 100
pgbench -c 50 -n 10000
? Specifically transactions per second?