Any nvidia card will work for this. You can pick up a cheap 8800 gtx off ebay for $50 and hook up the 3rd or 4th monitor to that card.
I think this is right - I'm pretty sure you can drop in any other nvidia card if you only need it to drive a desktop monitor. I've never done it myself, but I don't think you need to add another 580 to do it.
If you want SLI 580s, any 2nd 580 will enable you to drive 3 screens.
A 750w PSU should be fine for 580+8800GTX