Couple of things to consider, resale value on the 290s is not going to be great, however it will go some ways to helping with the cost of a new gpu.
DX12 is coming, it natively has support for multi gpu's, now this means it will be easier for devs to harness more than 1 gpu, it still requires work from the devs to implement however, but could help some ways towards xfire / sli setups.
Your 290's will be more than capable for a while longer yet.
However if you are going down the single gpu route, you are going to need to buy either a 980ti, TitanX or a FuryX, and none of them as a single card has the potential to be better than your 2 x 290's when working in xfire perfectly.
What monitor do you have? if you could get £250 or so for your cards thats half the price of a FuryX right there, while its not going to be better than your 2 x 290 cards, its going to be miles better than a single 290, and especially good at higher resolutions.
alternatively i would suggest the 980ti, the aftermarket ones are beasts like the G1, although not cheap at all, however are currently trading blows and often beating the furyX in single card scenarios.
I will say though, the FuryX is brand new and as a result has not had much in the way of drivers or any form of programs to unleash its overclocking potential.
If it was me? i would sell the 290's and buy the FuryX, its got an AIO bolted on which means extremely low noise and cool performance, and its got a lot of potential for overclocking. But a strong argument can be made for the 980ti as well.