high transaction rate

Hi,

I need to tune my database for a high update rate of a single small table.
A little simplified it looks like this:

CREATE TABLE temp_agg(
topic TEXT PRIMARY KEY,
tstmp TIMESTAMP,
cnt BIGINT,
sum NUMERIC
)

The table has 500 rows.

A transaction looks simplified like this:

1) select * from temp_agg where topic=$1 for update

2) if date_trunc('second', tstmp)=date_trunc('second', $3) then:
2a) update temp_agg set cnt=cnt+1, sum=sum+$2 where topic=$1
2b) set local synchronous_commit to off

3) if a new second has started:
3a) insert into other_table select * from temp_agg where topic=$1
3b) update temp_agg set tstmp=date_trunc('second', $3) cnt=1, sum=$2 where
topic=$1
3c) emit a notification (pg_notify(...)) with the new data

As a first test my program starts 10 writers each of which serves 50
topics. The timestamps are generated in a way that on average 10 timestamps
per second per topic a