Disabling case sensitivity

Hi

I would like to know if there is some way how to disable case
sensitivity in PostgreSQL server. With some varaiable or some setting
in configuration file  or how?
So after this would be the same 'Good' and 'GOOD' or 'GoOd' in some
SELECT from table where username='Good'. I wanna get the same row
with:
SELECT from table where username='GoOd';
Exactly like in MS SQL server.

Thanks a lot for any help