How to develop a plugin that uses 2 databases ?

Hi,

As you can see from my post count i am very new to the wordpress & forums, so i apologize upfront if such an issue has been posted already.

I have the following problem. I want to use a wordpress site as a front-end for user registrations, login and user management for a specific service. In order to do that i first need to develop a wordpress plugin that accesses 2 databases (one for the wordpress-related stuff, and other as for the service and user database).

Note that i cannot collocate databases as they are on different machines (different IPs), and that wordpress DB uses mysql 5.0.77 while service and user DB uses postgress 8.1.21.

I would appreciate some tips on how to do it, and what is the best approach. Any links, tutorials, and advice in general will be appreciated.