Xbox One X will support 1440p
The Xbox One X will support 1080p, 1440p, and 4K resolutions, partner group programme manager for Xbox Kevin Gammill said on Twitter. Gammill said that when a game runs at 2,560×1,440, the Xbox One X will natively output the resolution to supported monitors. This means gamers who have a 1440p monitor and connect their Xbox One X to it will get the full benefit.
Should the console be connected to a 4K TV, the 1440p resolution will be scaled up to 2160p.
On a full HD TV, it will be super-sampled and output.