Best nvidia GPU, with low power consumption?

I am planning on buying another referb server. It will come with some old GPU, just good enough to power a monitor, but that is about it.

So I would like to replace the GPU with something that will crunch. I don't crunch with GPUs much any more, but want the ability when needed for a challenge or a new app or project. The goals:

1) nVidia, so that it will crunch the widest range of GPU apps
2) Low power consumption while idling. My servers run headless, if that matters.
3) Good performance vs. power consumption when crunching. I know this is kind of a subjective thing. Trying to keep the power bill small, as much as possible. But getting tasks crunched is also the goal.
4) Lower cost is better. Not looking to spend $500+ on this GPU.
5) It can be double wide.

GTX 1070

http://www.newegg.com/Product/Produc...09%20601202919