It depends on the task. Results for AE are very different, the *only* card from the 700 series that always
beats the 580 for any test is the 780 Ti. Check reviews (I have, extensively, and I've tested a lot of cards).
For CUDA, a 980 is slightly slower than two 580s, about 10% less, and I was testing with a good 980 (1266MHz core).
Thus, a 970 will be a bit slower still.
Hence, a 970 should be faster than one 580 for CUDA, but make sure the app supports Maxwell V2 CUDA, because
not all apps do yet (AE doesn't).
Also, performance isn't purely related to absolute throughput. Indeed, one reason why the 580 is strong for CUDA
is that its high bw is shared across fewer cores, providing a lot more bw per core. Other reasons include a 2X higher
shader clock than later cards, etc.
The down side of course is power consumption, noise & heat. My 980 is very quiet, whereas my quad 3GB 580 CUDA
research box is impressively loud under load.
CraigN, don't compare or assume anything on a basis of assumptions vs. the 600 series, as the 6xx
cards were pretty terrible for CUDA. Check the
AE benchmark thread by Teddy Gage, the 600s and most
700s get hammered. Of the older cards, the only one that's usefully better overall than two 580s is a
single 780 Ti (sorry Omar, you're wrong). For the newer 900 series, the problem is app support for V2 CUDA.
Check the Arion page, my quad-580 is system is no. 17, scoring 5407; by contrast, my single 980 scores 1957.
NB: system 15 on the Arion page has a typo, it should read as a triple-780. The page author hasn't fixed it yet.
Btw, Teddy's very first post on the AE thread shows his 580 beating his 680.
Note that for
gaming of course, a 970 will leave a 580 in the dust, and indeed in many cases will likely be
faster than two 580s. I've done a fair few tests, comparing a 980 to one, two and three 580s, results on my
site.
Check the Unigine Heaven/Valley tests, CoJ and Stalker/FC2 at high-res.
Ian.
PS. Don't take my word on all this, check toms' own review:
http://www.tomshardware.com/reviews/geforce-gtx-760-review-gk104,3542-19.html
PPS. To the original poster, a 970 will be faster than one 580, but will definitely not be
faster than two 580s. Thus, it depends how much you care about noise, power, heat, etc.,
as the 970 will certainly be better for environmental factors. Actually, 300 UKP should be
enough for three 1.5GB 580s, or at least two 3GB 580s these days.