What does SLI ready mean and how to use it

For going multi-graphics cards, you need to ascertain these things,

• Does your motherboard support 3 graphics cards?
  • I think each would be x16
  • given your question & PSU detail, I guess you have a good SLI capable board
  • SLI PSU ref
• Will your power supply unit take the three cards?
  • this is in terms of wattage and connectors
  • I get the feeling that @1200W, you probably have the connectors too, but worth checking
• Once you get these things sorted out, the next important angle is the thermal one
  • Do you think you have enough cooling and circulation to handle three heat producing elements kept close to each other?
• Do you have a multi-monitor setup? or, a high resolution (beyond 1600 horizontal) display?
  • Most of the configurations are not worth unless you have all that visual real estate
  • look at the SLI FAQ on multi monitor setup to figure out your display setup
• Will you be utilizing all this effort with the applications you use?
  • Are the games you play capable of using all the hardware that you plan (look at the game specs and reviews)

I put up some references in an earlier answer to
Is it possible to run two separate video cards? yesterday.

Update: 3-Way SLI Gets PhysX Debut with Cryostasis and references within.

Legit Bottom Line: A dedicated NVIDIA PhysX card improves performance across the board and a pair of GeForce GTX 280 video cards in SLI with a dedicated PhysX card performs better than a set of GeForce GTX 280 in 3-way SLI!

No, you cannot. nVidia cards are limited to driving 2 independent displays per master GPU. When you enable SLI, one GPU becomes a slave to the other, and is no longer capable of driving independent displays.

With SLI disabled, you can power/drive all four monitors just fine.

If you enable SLI however, two will be shut off. You can get one to turn back on if you set the cards to run the monitors in Surround 2D or Surround 3D, but then you lose the benefits of them being separate monitors - windows simply sees the setup as one triple-wide monitor - one independent display (but built off of three linked displays).

There's little reason to get two different cards - If you're not going to run SLI, might as well keep things simple and get two of the same. Then you can always SLI if you need that additional power for something.

What you can do, and I did until one of my cards died, is run 3 graphics cards, two the same, and one different. You can SLI the two similar cards, and use those to power two of the monitors, and then drive the remaining two monitors off of the third card. Because the third card is different & unable to SLI with the first two, it will not have any of its outputs turned off when you enable SLI. It doesn't even have to be an expensive one, unless you plan to run fullscreen applications on the monitors connected to it. I used a $25 card since I only used the monitors for webbrowsing / chat windows or similar, and you can drag windowed games onto the auxiliary monitors and they'll continue to run with SLI since that's all rendered off-screen and composited into place in most of today's operating systems.

You can also look into AMD cards, as they are capable of driving up to 6 displays off of a single GPU (provided correct connectors, usually you'll need a monitor with DisplayPort, or an adapter to convert from DisplayPort to what your monitor can accept), but I'm afraid my knowledge there isn't quite as extensive.

This answer is slightly out of date now. The GTX680 and later cards can drive up to 4 independent displays off of a single GPU, though I think it will only do 3 of the monitors in a nVidia Surround setup; the fourth is limited to being an auxiliary monitor, typical of a "3+1" setup.