Checkpoint Tuning Question

I'm running a steady state test where I am pushing about 600 queries
per second through a Posgres 8.3 system on an 8 CPU Linux system.
It's a mix of inserts, updates, and deletes on a few tables - the two
biggest ones probably have about 200,000 rows.

Harddrive is just a simple, run-of-the-mill desktop drive.

Here are parameters that I have changed from defaults:

shared_buffers =100MB
synchronous_commit=off

And, after noting complaints in the log file about checkpoint intervals, I set:

checkpoint_segments=10

Then I turned on slow query logging for queries that take more than 1
second, and checkpoint logging.

Typically, I see no slow queries.  The system handles the load just fine.
Once in a while, I'll see a query that takes 3 - 5 seconds.

However, once the checkpoint process begins, I get a whole flood of
queries that take between 1 and 10 seconds to complete.  My throughput
crashes to near nothing.  The checkpoint takes between 45 seconds and
a minute to complete.

After the checkpoint completes - the system returns to having very few
slow queries, and the keeps up with the load fine.

Is there anything I can do to prevent the occasional slow query?

Is there anything I can do to prevent (or minimize) the performance
impact of the checkpoint?

Thanks,

Dan