Maximum transaction rate

I'm using postgresql 8.3.6 through JDBC, and trying to measure the maximum
transaction rate on a given Linux box. I wrote a test program that:

- Creates a table with two int columns and no indexes,

- loads the table through a configurable number of threads, with each
   transaction writing one row and then committing, (auto commit is
   false), and

- reports transactions/sec.

The postgres configuration regarding syncing is standard: fsync = on,
synchronous_commit = on, wal_sync_method = fsync. My linux kernel is
2.6.27.19-78.2.30.fc9.i686.

The transaction rates I'm getting seem way too high: 2800-2900 with one thread,
5000-7000 with ten threads. I'm guessing that writes aren't really reaching the
disk. Can someone suggest how to figure out where, below postgres, someone is
lying about writes reaching the disk?

Jack