You definitely should go with the HD7870 ghz. They're all very power efficient and quiet, plus lots of overclock headroom too if you're in to that kind of stuff. Based on this review "http://www.techpowerup.com/reviews/NVIDIA/GeForce_GTX_680/28.html"
The HD7870 ghz beats every other card out there for Performance/Watt, even the GTX680 is beat by double-digits.
And if i can recall correctly, i don't have a source though, since Catalyst 12.11 some reviewers described the HD7870 ghz as "a total new card" with performance boost of 10-20% in some games!!! I'm pretty confident to say that the HD7800s GPU should be the gamer's choice as the HD7900s are more of a compute-enhanced. AMD is making the same mistake as Nvidia did with their GTX580 (GF110), wasting transistor and space for compute engines. It's funny how both the Pitcrain and Taithi both have 32 ROPs so technically the HD7900s are being slightly bottlenecked. In terms of GPGPU, don't worry, even though it has been Double-Precision crippled, it has higher DP Flops than GTX680 (lol!)
reason why i kinda of promoting the HD7870 ghz so much is purely performance/watt and performance/price rato! (perf/watt king is Titan i think...)
You can surely pm me if you wanna have some more info!