In terms of overall gaming performance, the graphical capabilities of the AMD Radeon R7 260X are very slightly better than the AMD Radeon HD 7790.
The Radeon R7 260X has a 100 MHz higher core clock speed and the same number of Texture Mapping Units as the Radeon HD 7790. This results in the Radeon R7 260X providing 5.6 GTexel/s better texturing performance. This still holds weight but shader performance is generally more relevant, particularly since both of these GPUs support at least DirectX 10.
The Radeon R7 260X has a 100 MHz higher core clock speed than the Radeon HD 7790 and the same number of Render Output Units. This results in the Radeon R7 260X providing 1.6 GPixel/s better pixeling performance. However, both GPUs support DirectX 9 or above, and pixeling performance is only really relevant when comparing older cards.
The Radeon R7 260X was released less than a year after the Radeon HD 7790, and so they are likely to have similar driver support for optimizing performance when running the latest games.
The Radeon R7 260X has 1024 MB more video memory than the Radeon HD 7790, so is likely to be much better at displaying game textures at higher resolutions. This is supported by the fact that the Radeon R7 260X also has superior memory performance overall.
The Radeon R7 260X has 8 GB/sec greater memory bandwidth than the Radeon HD 7790, which means that the memory performance of the Radeon R7 260X is marginally better than the Radeon HD 7790.
Both the Radeon R7 260X and the Radeon HD 7790 have 896 Shader Processing Units. The two GPUs are based on different architectures, but deliver an equivalent shader performance. To compare, we must continue to look at the memory bandwidth, Texture and Pixel Rates. In this case, the Radeon R7 260X has 5.6 GTexel/s better Texture Fill Rate, 1.6 GPixel/s better Pixel Fill Rate, and 8 GB/sec greater memory bandwidth, so should have a marginally faster performance than the Radeon HD 7790.
The Radeon R7 260X requires 115 Watts to run and the Radeon HD 7790 requires 85 Watts. We would recommend a PSU with at least 450 Watts for the Radeon R7 260X and a PSU with at least 500 Watts for the Radeon HD 7790. The Radeon R7 260X requires 30 Watts more than the Radeon HD 7790 to run. The difference is not significant enough for the Radeon R7 260X to have a noticeably larger impact on your yearly electricity bills than the Radeon HD 7790.