Step 1: Install your hardware.
Step 2: Boot up and get x going
sudo dpkg-reconfigure -phigh xserver-xorg
(if you're already root, don't use sudo)
use the wizard to set up drivers for whatever monitor the wizard is showing up on. (usually the first on the bus)
reboot to start x
Step 3: Check the order and find the addresses
open a terminal and type
you should now see a list of devices and their bus id. find the graphics cards and write down the bus id's you'll need them later.