A 290x is about equivilent to a 970, so you could add a second and save the cost of a card. If that isn't an option I would go for the 980TI because it has 6GB Vram compared to the 4 or 3.5 on the others. I have a pair of 970s, but would not reccomend a multi gpu setup unless you can only budget for one card now, and another later one. Since you have a 290x and you are considering a fury x or 980ti that is not the case.
What power supply do you have now?
The best option would be to wait until new cards are released this year and get one of those.
just save your cash and wait for the new GPUs which will be able to push 60fps at 4k at the high end, or should be able to anyways , and also look to see if nvidia fixes it's DX12 shortcomings on their next gen cards
Thx guys I'll just wait then. The reason I was gonna go 970 SLI was cuz it's pretty much a 980 ti on benchmarks and more power effecient. I did PSU calculations and the recommended setup for my rig for crossfire 290x puts it up to 1000-1200w usage, whereas the SLI 970 would only put me at a single 290x usage. I have a 760W PSU, so a corsair 1200 axi would set me back another 200 dollars.
If i was you and wanted to be frugal (and I kinda was a year back or so) I would probably get a 390X or just be happy with the 290X. If I had too much money I would get the Radeon Pro Duo next month ;) Or probably just a couple of 980ti's.
A 390 crossfires happily with a 290. X or no X makes no difference, it's not like with Nvidia when they need to be the same to even consider SLI. Sure you'll be VRAM limited by the 290, really depends on what kind of resolutions you use, at 1080p 4GB is more than enough. I have a 290X and a 290 on a X79 2011 system and use a good 860 Watt PSU, a decent 1000W is more than enough. Frame pacing for DX9 is working in crossfire, most DX9 games I've tried I get ~90 FPS in 4K. Skyrim depends heavily on what ENB you use.
Which brings me to that I also agree with what others in the thread have said, Crossfire/SLI si not for the faint of heart. There will be issues, and some will not be workable depending on your preferences. One fast card is less of a headache.
Nitpick: The 290X actually not really comparable to a 970, I have mine overclocked at ~1100 Mhz, and no, a 970 is not as fast. At 1100 Mhz a 290X is noticeably faster than a 970 at 1400 Mhz. Hawaii has higher "IPC" so to speak compared to Maxwell, it gains more from a frequency increase. The drivers have made good increases over the last year or so, and of course in DX12 Hawaii/Grenada kinda shines. Maxwell is at best almost as fast in DX12 as in DX11, as they can't replace the shaders with their own in DX12 they tend to be a tad slower in the latter.