I would wait the geforce 1000 series to arrive around spring or summer. Current cards are very overpriced and its unknown if the upcoming Dx12 cards will be big step forward. E.g. that GTX970 is as fast as my GTX780 and I bought it two years ago with same price that GTX970 have today (~400euros). GTX 980Ti is superb but not sure if very worth of the price. DX12 cards may cut prices much down and offer better choises.
For me i7 5820K and 32Gt DDR2400 memory gave nice boost for UE4 editor. Would start upgrading CPU/memoery and then bit later get the 3d card.
Edit: Yes, DX12 is already here, but the 1000 serie will have much new tech. Interesting to see how those will play and affect pricing. Else would also be interested of GTX 980ti, it just should be half less costy.