Don't go for SLI. Save money now and get a better Single GPU in future, GTX 580 is enough to handle games out now.
Wait for the GTX 700 & Radeon 8000 series. They'll be out early next year.
I understand the concern with the 700 series being possibly only 3-6 months away. They would in fact still be using the Keplar Architecture with possibly a increase of 20-30% and thats just from what i've seen from older generations of GPU's(480<580) using the same Architecture with optimizations of course. I was hoping to wait for maxwell to come out in 2014, it seemed like a much more value orientated decision by combining another 580 to last me till then, but i will keep that in mind. Thanks for the reply.