One final question - can I plug in an old PCI video card to drive a fourth display (obviously separate from the eyefinity config)? Or rather will eyefinity conflict with it in any way? What would happen if I moved a video between it and the other displays (assuming software rendering I guess)? The card is a super old ATI Rage card (haha).
Assuming that the drivers don't conflict I don't see why you would have a problem.
When I initially got my displays I ran an 8800GTS (320) with an ancient (well, more ancient) 5200 FX. Used the same drivers and worked like a charm.
Video should work seamlessly. Note however that if you choose to use something like SoftTH to render across all four monitors fullscreen you will run into bandwidth issues. For me the PCI bus was limited to ~20 fps, no matter what I was trying to do (be it Half Life 2, video, or Pong: 20 fps)