R9 270x is a rebadged 7870. Its sufficient for your resolution and gaming needs.
For info.,lets compare them.
R9 270x vs HD 7870
The R9 270x is 8% better performer than 7870. (source- game-debate.com
In terms of overall gaming performance, the graphical capabilities of the AMD Radeon R9 270X are very slightly better than the AMD Radeon HD 7870.The Radeon R9 270X and the Radeon HD 7870 have both the same core clock speed and the same Texture Fill RateThe Radeon R9 270X was released over a year more recently than the Radeon HD 7870, and so the Radeon R9 270X is likely to have better driver support, meaning it will be more optimized for running the latest games. The Radeon R9 270X and the Radeon HD 7870 have the same amount of video memory, but are likely to provide slightly different experiences when displaying game textures at high resolutions.The Radeon R9 270X has 25.6 GB/sec greater memory bandwidth than the Radeon HD 7870, which means that the memory performance of the Radeon R9 270X is slightly better than the Radeon HD 7870.
Therefore, you must buy R9 270x.