As of Postgresql version 7.0.2, there really isn't much reason to use mysql anymore. If you site is slow, switch your webserver to thttpd first, then put a squid server in front of it, THEN look making the database run faster.
I've found Postgresql very stable and very capable, and for the stuff most people do with mysql, I do with gdbm, which is much faster than mysql for lookup tables, and do all my real dbms work in postgresql now.