SLI & Crossfire is still a bad idea and a waste of money imo, just get the best single gpu car not only will you save money but you will also save some headaches.
When I first built my system 2 years ago, I built it thinking I would add a second 580, but now I'm going to sell it and get a GTX 770
SLI/Crossfire have some issues yes but i wouldnt say its waste of money. I think it really depends are you buying whole new computer or just making some update. For example, im running gtx680, so smart for me would be buy another 680 atm since they are quite cheap (under 300euros). 2x680 in sli will get really good performance. Ofc it wouldnt help for coh2...