I am looking to build a new pc for use with blender but i am not sure whether a gtx 970 or 2 760s in SLI will give better performance. Both options are about the same price. Help?
Hi, two 760 are faster than one 970.
Are you compare the prize 970 with two 4 GB 760? GPU memory is not added for render engines and SLI can not used for Cuda applications like Cycles.
Cheers, mib
the sli part puzzles me a bit, i yet to lay my hands on any sli setup, jsut wondering how do two cards work with cycles, do you have to run 2 blender instances in the same time in order to get advantage of both cards?
Two 760s are faster, but don’t put them in SLI mode. SLI kills compute performance.
wgan, to run 2 cards go to user preferences > system > Compute device and select GTX XXXX x2. And no you only need run 1 instance of blender
Just to add some more info about what happens behind the scenes… 
For two cards Blender copies the cycles kernel to each card along with the scene meshes/textures and starts the render. At regular intervals Blender then copies the raw images from each card, combines them with what has already been rendered, and then displays the results to you the user. Each card is used like a separate machine to render the image. In this way you should be able to use as many cards as your computer can hold.
SLI mode allows the two cards to share information but only if the application is written to use it. Cycles like most GPU render engines doesn’t know anything about SLI like games do, so it gets in the way if enabled. That is why you shouldn’t use game performance tests to determine a good render card. For games the GTX980 can outperform a GTX780ti but it doesn’t come close to the same performance with GPGPU computing.