OK, so this was utterly painful and I needed to share what finally worked for me. I have 2 x Nvidia GTX480 cards. I have 3 x monitors. I wanted to be able to run all three off the same box, using it as a single X Screen (i.e. so I can move windows between screens and it acts correctly). No matter what I tried, it wouldn't let me do so with the proprietary driver. I asked on Stack Exchange and got somewhere closer.