Twin Nvidia graphics cards and Ubuntu
My home desktop PC, running Ubuntu, has had two monitors for quite some time, but when an obliging friend on IRC offered me a third, I couldn't pass it up.
This meant installing a second graphics card, and fortunately I had a second Nvidia GeForce 9400GT (PCI Express), identical to the one already fitted. This went in perfectly but wasn't recognised by anything. When I took the cover off again and looked more closely, it seems there's a jumper which needs rotating to switch into 'dual video cards' mode:
Having done this, everything sprang into life. Tell Nvidia's settings applet to overwrite the existing Xorg.conf file (not merge with it), set them all as separate X displays, and Ubuntu spreads cleanly across three of them. Shame I don't have a fourth...