Replacing old tuples on insert

Hi all,

I have not subscribed to this list. However I have registered to the
list with another id of mine which is not working right now.

I have a table as follows -
table TrendData( TrendID bigserial,
                         TrendName varchar(30),
                         .....
                         .....);

Now for this table i want a maximum of only 5000 tuples. After the
maximum is reached the new tuples that are inserted should replace the
older ones. Could anyone help me out in writing the query or a trigger
also would be fine.

Thanx and Regards,
Pradeep