I want to note one thing before proceeding to the tests. When benchmarking my Radeon HD 5870 or Radeon HD 5870 CrossFireX on the platforms with CPU frequencies of 2.67GHz, I saw in some games that the average and bottom frame rate was higher at 1920x1200 than at 1280x1024. This is not an error in the diagram, but a real fact. I can’t give you a thorough explanation but I am sure that when such a thing happens, it means that the graphics subsystem is too fast for the given CPU or platform at large.
In the diagrams below the results of the Intel Core 2 Duo platform are marked orange, those of the AMD Phenom II X4 platform are marked green, and those of the Intel Core i7 platform are marked blue. Semi-synthetic benchmarks go first.
The Radeon HD 5870 is clearly CPU-dependent, both singly and in CrossFireX mode, in the good old 3DMark 2006. The only exception is the high-quality test mode at 1920x1200 where the single card does not speed up as fast as in the other three cases.
Talking about the platforms and CPUs, the Intel Core i7 is expectedly in the lead. Clocked at 3.4GHz, it is successfully competing with the 4.1GHz Phenom II X4. Although the Core architecture and the Intel Core 2 Duo processor are over 3 years old already, the dual-core CPU (when properly overclocked) is quite competitive to newer and more expensive CPUs.
The newer 3DMark Vantage relies less heavily on the CPU, yet the Radeon HD 5870 and its CrossFireX configuration lack CPU resources in the Performance mode. This is clearer with the dual-card configuration, of course. As for the CPUs and platforms, the Intel Core i7 is in the lead again whereas the Intel Core 2 Duo is not as confident as in 3DMark 2006.
Unigine Heaven Demo
No comments about the CPU-related limitations. But I can note that the CrossFireX technology brings about the same performance gain on each of the three tested platforms.