A CPU for Dual GPU Gaming: i5-2500K or FX-8350
Looking back through the results, moving to a dual GPU setup obviously has some issues. Various AMD platforms are not certified for dual NVIDIA cards for example, meaning while they may excel for AMD, you cannot recommend them for team Green. There is also the dilemma that while in certain games you can be fairly GPU limited (Metro 2033, Sleeping Dogs), there are others were having the CPU horsepower can double the frame rate (Civilization V).
After the overview, my recommendation for dual GPU gaming comes in at the feet of the i5-2500K. This recommendation may seem odd – these chips are not the latest from Intel, but chances are that pre-owned they will be hitting a nice price point, especially if/when people move over to Haswell. If you were buying new, the obvious answer would be looking at an i5-3570K on Ivy Bridge rather than the 2500K, so consider this suggestion a minimum CPU recommendation.
On the AMD side, the FX-8350 puts up a good show across most of the benchmarks, but falls spectacularly in Civilization V. If this is not the game you are aiming for and want to invest AMD, then the FX-8350 is a good choice for dual GPU gaming.