In terms of overall gaming performance, the graphical capabilities of the AMD Radeon HD 7970 GHz Edition are marginally better than the Nvidia GeForce GTX 770.
The GeForce GTX 770 has a 46 MHz higher core clock speed and the same number of Texture Mapping Units as the HD 7970. This results in the GeForce GTX 770 providing 5.9 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 GeForce GTX 770 has a 46 MHz higher core clock speed than the HD 7970 and the same number of Render Output Units. This results in the GeForce GTX 770 providing 1.5 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 GeForce GTX 770 was released less than a year after the HD 7970, and so they are likely to have similar driver support for optimizing performance when running the latest games.
Both GPUs exhibit very powerful performance, so it probably isn't worth upgrading from one to the other, as both are capable of running even the most demanding games at the highest settings.
The HD 7970 has 1024 MB more video memory than the GeForce GTX 770, so is likely to be much better at displaying game textures at higher resolutions. This is supported by the fact that the HD 7970 also has superior memory performance overall.
The HD 7970 has 63.7 GB/sec greater memory bandwidth than the GeForce GTX 770, which means that the memory performance of the HD 7970 is much better than the GeForce GTX 770.
The Radeon HD 7970 GHz Edition has 2048 Shader Processing Units and the GeForce GTX 770 has 1536. However, the actual shader performance of the HD 7970 is 1828 and the actual shader performance of the GeForce GTX 770 is 1667. The HD 7970 having 161 better shader performance and an altogether better performance when taking into account other relevant data means that the HD 7970 delivers a significantly smoother and more efficient experience when processing graphical data than the GeForce GTX 770.
The Radeon HD 7970 GHz Edition requires 250 Watts to run and the GeForce GTX 770 requires 230 Watts. We would recommend a PSU with at least 500 Watts for the HD 7970 and a PSU with at least 550 Watts for the GeForce GTX 770. The HD 7970 requires 20 Watts more than the GeForce GTX 770 to run. The difference is not significant enough for the HD 7970 to have a noticeably larger impact on your yearly electricity bills than the GeForce GTX 770.