I were given two 580 GTX evga cards and thinking of putting them to use. Should I use SLI or not? I know that sli makes two card work as one etc. Hear some say SLI is not good for cycles, better to run them without the bridge.
Short answer: turn it off when rendering with cycles. My video card is almost the same as yours (evga 590gtx, its a dual proc card, so the computer detects it as 2 video cards) and runs faster with cycles when not on SLI mode.