560 is no good for mining how every 580's are expensive but if u get 8gb one u can duel mine and ROI will be fast
560 are the best card for mining at the moment du to the price of 570/580
560 can achieve 15MH/s and more on ETH and more than 470H/s on Cryptonight (Depending of Memory and strap). Branko was the first to explain that.
I have 560 Micron memory Hash 15.6MH/s for less than 40W and Hynix 15.2MH/s