I was concidering getting a 2nd 6970 to crossfire, but a friend said to just to get a 7970. I am currently using a 8150 fx, with a ASUS Crosshair V Formula-Z AM3+ AMD 990FX (I was also thinking about getting an 8350, or possibly switching to an i7 if someone has some recomendations). I am thinking of setting up eyefinity, on 3 ASUS VS248H-P Black 24" monitors.
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.
[i had the exact same question about 2 months ago, however I had just sold my xfx 5970 limited card. I chose the 2x msi r6970's which were used and at a very good price.
The cards are letting me run max Payne as well as far cry on ultra as you have 4gig of ram available.
The draw back is the heat of the cards. They do run hot so you will need to make the ventilation in your case is good. There is a power draw too but that you know I'm sure. I would agree to wait until the next gen cards come out but the crossfire is also a pretty sweet option.