What are the best graphics cards for cycles? I'm considering having a system build solely for rendering for cycles (as kind of a single PC renderfarm). Since the GPUs would be most important here, I plan on having it build around those. I was wondering what would be the best GPU to go for? As far as I can find, 2 gtx 580's seem to be the best option, while two 560 ti's give more bang for the buck. Any chance newer videocards work as well? (600+series?)
Here's a spreadsheet that collates data from many users running the same benchmark scene. The thread with active discussion and a copy of the scene is at BlenderArtists, This is the best we can do without relying on opinion.

Decide for yourself which gfx card makes sense. 

Note: Different OS's, drivers and versions of Blender, can all significantly change render times. So you'll need to take that into consideration when comparing benchmarks.