GTX670 SLI
NVidia's Cost to performance really sucks at the moment with the GTX780 and Titan
The GTX770 will be releasing soon for the same spec as the GTX680 but a little cheaper. I'm sure at the point the GTX titan and GTX780 will also lower their price. For now, people will continue to pay inflated prices.
Anything like Tri-SLi or Quad-SLI, I would then sell your cards for an upgrade. Tri-SLi and Quad-SLi drivers really suck