Storing a file hash as primary key

I was wondering what the best way is to store a file hash (MD5 or SHA1)
and make it primary key indexed.
I have seen some people storing the hexadecimal encoded MD5 in a
CHAR(32) but it may be a better idea to use a CHAR(16) without encoding
the string, but that may cause some problems.

What do you recommend?
Do you have any experiences storing file hashes in a database?
Do you know any good opensource software that stores file hashes in the
database (to take a look)?