Add fields into mysql WP_USERS and Profiles

I need to add some fields into Users Profile. For example: date_of_birth

I know php/mysql well, but i’m loosing time just looking for files i get to “hack” it.

Could some one please tell me which files i should change for that ? or, if there’s some plugin for that, good to .. hehe

About plugin, i already try Register Plus, but it’s not working well, with lots of bugs.

thanks.