LISTEN question

Hi !

Does any one have any idea on the impact of using a large number of
different LISTEN,
say that I add 20 "LISTEN something;" statements will this slow down the
client much ?

The thing is that I have 20 different tables and I would like to be notified
when something
change in any of the tables, there are timestamps so I can use SELECT * FROM
xxx
WHERE timestamp>zzz to get the changed data but going to put out 20 SELECT
queries all the time does not sound very smart so I thought I would use
LISTEN/NOTIFY instead to improve performance.