Basically what the title says. should i get two gtx 780, two r9 290x or a radeon hd 7990. To me prices don't matter since all of these are roughly the same price. I will be gaming at 1440p and will mostly use my gpu for extreme gaming, video editing and for doing high end projects.
Do you care about screen tearing and stutter? Vsync input lag? If you don't care, go with AMD and hope freesync will work other than just on laptops.
If you want smooth gameplay even when the FPS dip, then go with Nvidia and check out their gsync tech.
The R9 290X has fixed [some] issues in CrossfireX by using the PCIe bus instead of a separate crossfire bridge. The HD 7990 is about the same as the R9 280X in crossfire as the R9 280X is a rebadged HD 7970. The GTX 780 competes with the R9 290 and gets beaten by the R9 290X. If you want to go Nvidia try the 780ti in SLI but they are generally more expensive. Also the Nvidia cards support G-sync with AMD cards supporting Free sync and Mantle. So take your pick.
In terms of performance: CF 290x > GTX 780 SLi > 7990. Note that AMD solutions run hotter than NVIDIA's. Also note, that NVIDIA has a better ecosystem/features than AMD has. Lastly, I would say away from 7990 (coil whine and high thermals) and reference design 290x (runs at 95C and sounds really loud). Third party cooler work pretty well on 290x, but will still run hotter than gtx 780.