So far UE4 doesn’t use more than 2 graphics card but with that being said
Go with the 290x or if you can get even better deal with the 290 go with that. Even if the 290x was around the same price I would say go with that, but with that price difference the 290x is the obvious choice. Just make sure it’s not the reference design which ran loud and hot, where as the aftermarket cards all ran silent and cool( something like the triX or lightning.)
I would be very wary about the 970 going forward with what NV has done with driver updates on previous generation cards, and how their cards performance has aged vs AMD 7xxx series on.
Edit: So what I would suggest is a single 290x or if you get better deal the 290. They will be great performance for the price for quite a while. New cards are coming out in a few months, big maxwell, and R9 3xx, but even then I don’t expect the price of these current cards to change much, these will just have a higher price than where these are at. Then if you want more performance you should be able to sell your 290x without losing much money at all, and go with one of the new cards.