I don't think so using char or varchar instead integer is good idea. It is hard to expect how much slower it will be than integer PK, but this design will be slower - impact will be more terrible when you will join larger tables. If you can, use ENUM types instead.
http://www.postgresql.org/docs/9.2/static/datatype-enum.html
CREATE TYPE mood AS ENUM ('sad', 'ok', 'happy');
CREATE TABLE person (
name text,
current_mood mood
);
INSERT INTO person VALUES ('Moe', 'happy');
SELECT * FROM person WHERE current_mood = 'happy';
name | current_mood
------+--------------
Moe | happy
(1 row)
PostgreSQL varchar and char types are very similar. Internal implementation is same - char can be (it is paradox) little bit slower due addition by spaces.