Your best play is to get one GTX 780 Ti. Then get your new higher resolution monitor, then get your second GTX 780 Ti.
850 watts is going to be at the bare minimum for a 295x2. Guru3d recommends 850-1000 watts.
http://www.guru3d.com/articles_pages/amd_radeon_r9_295x2_review,12.html
The GTX 780 Ti in SLI, on the other hand, will do very well with 850 watts. Guru3d only recommends 800 watts for those.
http://www.guru3d.com/articles_pages/amd_radeon_r9_295x2_review,12.html
At 1080p, the 295x2 is only going to be about 18% faster than a single GTX 780 Ti. At 2560x1600, that advantage is still only 33%. Not really worth the extra cost.
http://www.techpowerup.com/reviews/AMD/R9_295_X2/24.html
Basically, the 295x2 was made for much higher resolutions and multi-monitor setups. It uses a lot of power and is very inefficient at normal resolutions. In addition to that, it also has some flaws that include coil whine and the usual poor driver support that goes with all Crossfire setups.
http://www.techpowerup.com/reviews/AMD/R9_295_X2/30.html
Quotes:
"Also, at lower resolutions, 2560x1600 and below, scaling is slim in general, and I would not recommend the R9 295X2 or any other multi-GPU solution for those setups. A powerful single-GPU solution, like NVIDIA's GeForce GTX 780 Ti or Titan, will definitely do better at 2560x1600 while being more cost efficient."
"Another surprise is that the card still produces some coil noise (listen for the chirping noises in our thermal imaging video). While less pronounced than with the HD 7990, it is still there. After all the HD 7990 drama, I expected AMD to make absolutely certain it wouldn't happen again. Depending on the situation (FPS, game, and load), coil nose also changes in both volume and frequency, which, at times, makes it incredibly distracting and very difficult to ignore. I find the R9 295X2's acoustic footprint higher than what would be necessary, especially in idle. The card runs at noise levels comparable to other high-end cards that are obviously slower under load, but I feel AMD could have optimized things a bit better. With current drivers, the card has absolutely no fan control or fan speed monitoring, so you can't even just adjust things on your own."