Varchar pkey instead of integer

Hi,

I am currently designing a database and wanted to know something that may
sound trivial, but I thought its still good to confirm before dumping
millions of rows in it.

The design requires a few master tables with very limited rows, for e.g.
currency_denomination table could at the max have a few records like million
/ billion / crore (used in india) / lacs (india specific) and so on.

Now what I wanted to ask was whether its any different to have the
primary-keys in such master tables as text/varchar rather than integer ?
i.e. Can I use a character varying(10) and use the text 'million' /
'billion' instead of a serial / integer type ?

p.s.: I am not as much concerned with the size that it'd take on the data
tables, as much as the fact that the select / insert performances shouldn't
suffer. However, if that increase in size (per data record) may make a
considerable impact on the performance, I would certainly want to take that
into account during design phase.

Any pointers / replies appreciated.

Regards,
*Robins Tharakan*