I would personally run SLI 780's. A single 780 will play most games on high to ultra high settings for a good while, depends on what you're playing. By that time, the 780 will drop even more, and I would just slap a second one in there for SLI for roughly a 20-50% increase (depends on the GPU and/or the game itself). Also, using SLI or Crossfire (AMD) will drop your PCIe speeds the more cards you install. Currently, the Z77, which is what I use, will still run dual x8's which is more than enough to keep up with graphics cards, so don't let the smaller number fool you. The third is when you get a x8/x8/x4, which the x4 may produce a slight decrease of the desired speed. The other best chipset option is the X79 which is the 2011 socket, which it stupidly expensive that can run SLI much more effortlessly, not sure on how many lanes exactly, and I don't want to give you false information, I recommend looking into that if you have not purchased a motherboard. And there may be more chipsets offering other SLI/Crossfire abilities, but the Z77 I am most familiar with because I own one, and I've came across X79's, but unfortunately I don't know enough about it, but it's all about personal preference, that's the fun of building a PC.
If you want the biggest and baddest rig that will play the highest setting games for years and years, given you have the money, go with triple Titan's, which is actually over-killing it.
Hope that was helpful. My highest preference in price/performance, would be to run SLI 780's ($650/each roughly), which is what I am currently in the making of doing myself.