Replacing old tuples on insert

Hi all,

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 would also be
fine.

Thanx and Regards,
Pradeep