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2 HD 6970s > HD 7970. If you can find a cheap used HD 6970 just grab it and CF it with your existing one. I don't recommend upgrading to a HD 7970 in any case, when you upgrade you should consider a 100% performance boost, adding in a HD 7970 should yield to a 30% performance increase over a single HD 6970, which means 10-15 more FPS in the demanding titles.
In case you CF HD 6970s, your CPU will be a bottleneck in CPU intensive games even if you upgrade to FX 8350, currently AMD CPUs not performing great like i7s in CPU intensive games, so if you crossfire you'll have excessive unused GPU power due to a CPU bottleneck in some certain titles.
Going Eyefinity will require you to upgrade almost each year to enjoy a smooth gameplay with good FPS in the demanding titles, so I recommend staying single 1080P right now.
HD 6970 is capable of running almost all the modern titles at good FPS, stick with it for a while at least until the current HD 8K series show up, then upgrade both the CPU and GPU.