Depends on your budget, and what you want out of your games. Do you like want to run everything maxxed out and 100+ fps or something a little less?
A single 590 would be okay, a pair of 560Tis if you want the SLi option at a budget. If not on a budget I would say a pair of 580s would be best.
Keep in mind, more video cards, means more room required, more heat, more power required, requires better cooling solution for your case, etc.... that all equates to more money too.
I've never been a big fan of 3/4way SLi, you see a decent loss at 3way and a bigger loss in performance at 4way. I could never see the appeal of paying for a graphics card that'll almost never be used more than half of it's potential, doesn't make sense for me, but that's just my option.
You can youtube a bunch of videos about SLi performance on games, and you'll see what I mean.
If one card can do the work of three then use the one card.
Your Crosshair V Formula will not really do full justice to 3 580GTX cards, the bottle neck will be quite intense based on my experience benching tri and quad SLI cards with a overclocked FX-8150 (6GHz on the CPU)
But if bling is the purpose then Tri-SLi rocks. Then go water cooled for the cards too and rule the house.
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Yea the Matrix GTX580s are almost if not now non existent. I think at the time I got them was the last time they were available as I had a pair on order for about 3 weeks before they came in. I'm so glad that I picked them up when I did. Their actually in numeric sequence by serial number which makes it that much more special to me.