Consolidation of Logins between 4 programs

I need to consolidate the logins between 4 programs, or have all 4 programs refer to the same database, I don't know the simplest and most reliable way to do it, that's your field... primary program: x-cart 4.0 noahs classifieds phpbb (latest version) copermine (gallery) the x-cart is sitting on one server and the other 3 programs are sitting on a different server .. the x-cart is the main program/site this is a start-up children's products site we want to add more content, so we are adding the message board, classifies & gallery program, but we do not want to have folks register 4 seperate logins, looking for the simplest most efficient solution for that, also looking for an on-going relationship for future tweaks to the various programs as we progress In a nutshell, create one registration login screen, result, access to all 4 programs