Several things have become very clear based on the benchmarks. To get the most out of a high-end single-GPU graphics card, you need an Intel Core i5 3570K or higher. So only if you really feel that you have to get the maximum frames per second, do you need to spend the money on a Core i7.
It's also interesting to note that AMD's low-end processors score better than Intel's cheap processors in combination with high-end graphics cards. That means you're better off with an AMD A6-5400K (about £56) or AMD A10-5800K (about £99), than an Intel Pentium G860 (about £52) or an Intel Core i3 (about £91). Particularly the Pentium and the Core i3 have a negative impact on the performance of the graphics cards.
The last conclusion we can draw is that the AMD Radeon HD 7970 GHz edition suffers less from CPU scaling than the Nvidia GeForce GTX 680. That means that the AMD card performs better with slower processors compared to the Nvidia card. We don't have a clear explanation for this, but it's possible it has something to do with the driver overhead.
So now you know. Should you upgrade your CPU before getting that high-end graphics card? Or do you instead go with an AMD card that runs better with slower processors?