From the reviews I've seen the 960 performs roughly on a par with a 760/670 in a lot of games so it'd give you a 20% or so uplift in performance.
For reference I ran Anandtech's GPU bench on that basis -
http://www.anandtech.com/bench/product/1350?vs=1038
If you can get £70 for your 580 then its an £80 upgrade and I'd say it'd be worth it. They'res a 4Gb gigabyte model I think was going for around £150-£160.
My own preference is for single cards - multi cards just seem like more hassle than they're worth