Single card will always give better performance, since there is no SLI profile to hamper overall quality initially. SLI can occasionally have issues with specific games and only use one card.

If you can afford the Titan X, get it. Later, you could always add a second in for more future proofing. Either way, game developers will find a way to make a port use 16gb of VRAM and render the cards less than perfect, but it is a long way off for now.

If you buy from EVGA, it comes with a 3 year warranty, and you can change things as long as you don't physically damage the card. You can add an all in one cooler, or Waterblock and not automatically void your warranty by removing the cooler.

If you purchase a used EVGA card from another user, the warranty transfers.

If you buy from Nvidia direct, it is a 3 year, non transferable warranty. I am not sure about changing things on it though.

It is expensive, and no matter the resolution you play at, it is also a great card. The higher the resolution, the more the card shines. 1440 is a minimum recommended, because the ROG Swift and other 144hz monitors at 1440 really look amazing.

That is assuming that DX12 will perform as well as they speculate. It will also only be for future titles and not help with anything released before dx12 is implemented.

Have game developers released dx12? I thought they were only using synthetic benchmarks currently. Didn't realize they had advanced to the stage of full production.