Well for $600-$700 you're in GTX 980 Ti territory. But how much different is a GTX 980 Ti as opposed to crossfire 290x? Well, a little and a lot.
http://www.pcgamer.com/nvidia-geforce-gtx-980-ti-review/
The 290x CF seems to definitely be the 1440p winner in average frame rates. But it suffers more in minimum frame rates. Then you have to consider games like Fallout 4 which run much better on Nvidia. So, is this a useful buy? Well, I'm not sure really, I suppose it depends on how much you get for your current cards.
If I were you I don't think I would do it (unless if selling the old equipment got me within $50 of the new card). Maybe wait a few months for Pascal to come out. But I just don't see that much of a difference between the...