IF you have multiple servers, the prefix would be a good idea if they all access the same database yes.
As for the second option, why would you need it if your database column is set to be unique? - You won't because it won't accept duplicate values (you'd need to use mysql_affected_rows() to see if it was accepted).
You can use the 2nd parameter if you want but if its only for password resets it's not going to be used as much as a normal login form. Once the user has reset you just wipe the unique key out of their record so you're not really going to need a super long unique string.