In terms of overall gaming performance, the graphical capabilities of the AMD Radeon HD 7990 are massively better than the AMD Radeon HD 7970 Crossfire.
The Radeon HD 7990 has a 25 MHz higher core clock speed and 128 more Texture Mapping Units than the HD 7970. This results in the Radeon HD 7990 providing 124.8 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 HD 7990 has a 25 MHz higher core clock speed and 32 more Render Output Units than the HD 7970. This results in the Radeon HD 7990 providing 31.2 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 HD 7990 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 Radeon HD 7970 Crossfire and the Radeon HD 7990 have the same amount of video memory, but are likely to provide slightly different experiences when displaying game textures at high resolutions.
The Radeon HD 7990 has 312 GB/sec greater memory bandwidth than the HD 7970, which means that the memory performance of the Radeon HD 7990 is massively better than the HD 7970.
Both the Radeon HD 7970 Crossfire and the Radeon HD 7990 have 4096 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 HD 7990 has 124.8 GTexel/s better Texture Fill Rate, 31.2 GPixel/s better Pixel Fill Rate, and 312 GB/sec greater memory bandwidth, so should have a significantly faster performance than the HD 7970.
The Radeon HD 7970 Crossfire requires 460 Watts to run and the Radeon HD 7990 requires 375 Watts. We would recommend a PSU with at least 750 Watts for the Radeon HD 7990, but we do not have a recommended PSU wattage for the HD 7970. The HD 7970 requires 85 Watts more than the Radeon HD 7990 to run. The difference is significant enough that the HD 7970 may have an adverse affect on your yearly electricity bills in comparison to the Radeon HD 7990.