In terms of overall gaming performance, the graphical capabilities of the AMD Radeon HD 7950 Vapor-X OC Edition are marginally better than the AMD Radeon HD 6950 Crossfire.
The HD 7950 has a 100 MHz higher core clock speed than the HD 6950, but the HD 6950 has 64 more Texture Mapping Units than the HD 7950. As a result, the HD 6950 exhibits a 40 GTexel/s better Texture Fill Rate than the HD 7950. This still holds weight but shader performance is generally more relevant, particularly since both of these GPUs support at least DirectX 10.
The HD 7950 has a 100 MHz higher core clock speed than the HD 6950, but the HD 6950 has 32 more Render Output Units than the HD 7950. As a result, the HD 6950 exhibits a 22.4 GPixel/s better Pixel Fill Rate than the HD 7950. However, both GPUs support DirectX 9 or above, and pixeling performance is only really relevant when comparing older cards.
The HD 7950 was released less than a year after the HD 6950, 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 6950 has 1024 MB more video memory than the HD 7950, so is likely to be much better at displaying game textures at higher resolutions. This is supported by the fact that the HD 6950 also has superior memory performance overall.
The HD 6950 has 80 GB/sec greater memory bandwidth than the HD 7950, which means that the memory performance of the HD 6950 is much better than the HD 7950.
The Radeon HD 6950 Crossfire has 2016 Shader Processing Units and the Radeon HD 7950 Vapor-X OC Edition has 1792. However, the actual shader performance of the HD 6950 is 1048 and the actual shader performance of the HD 7950 is 1452. The HD 7950 having 404 better shader performance is not particularly notable, as altogether the HD 6950 performs better when taking into account other relevant data.
The HD 7950 transistor size technology is 12 nm (nanometers) smaller than the HD 6950. This means that the HD 7950 is expected to run slightly cooler and achieve higher clock frequencies than the HD 6950. As they exhibit similar graphical performance, the HD 7950 should consume less power than the HD 6950.
The Radeon HD 6950 Crossfire requires 400 Watts to run but there is no entry for the Radeon HD 7950 Vapor-X OC Edition. We would recommend a PSU with at least 500 Watts for the HD 7950, but we do not have a recommended PSU wattage for the HD 6950.