I'd actually wait for AMD's 20nm R9 390/X graphics card to come out, that'll be something very much to consider.
For the moment, there is no real point in getting more or another card unless you have a resolution greater then 1920x1080 (2560x1080 and 1920x1200 as well shouldn't be an upgrade also). If you run QHD(2560x1440)/UHD(2840x2160) resolutions then it would be worthwhile.
I am currently running an Ultra HD display with an R9 290X and it does ok, but I am not willing to buy another R9 290/X (both will work in Xfire) when next gen AMD cards are going to suffice ultra HD much better.