It may look close, but really it's NOT.
PERFORMANCE: GTX 680 is a faster card.
It's faster at 1080p, a tad faster at 2560, and at multimonitor resolution.
Overclocking is a luck-based attempt, which generally closes the gap , but then you are left with not so stellar perf/W and the jet-like noise from 7970.
7970 is faster card only in cases where bandwidth _is_ an issue for 680,
where there are no CPU bottlenecks whatsoever, and CPU is largely irrelevant - Metro 2033*** being the perfect example.
So right away you can say that 680 will rule the day in any reasonably well coded multiplayer,
because of clients overhead, and 680's graphic workload optimized scheduler.
(***BEWARE: developer's beta of Metro: Last Light has already been seen running on Nvidia hardware and featuring some impressive PhysX/Apex Clothing effects)
PRICE: Wash!
GTX 680 is $50 cheaper, but 7970 comes with 1GB of VRAM, 1 VRM phase more, is sturdier built, has better PCB, and larger bill of materials.
POWER CONSUMPTION: 680 wins again. It depends on a game, but on average 680 will consume something in between of Tahiti Pro/XT.
They are pretty much the same on Desktop, with 7970 winning the day if you tend to leave your PC idling.
NOISE: Nvidia has far mor experience with fighting the heat, and this shows on cooler design.
10db of load difference as measured by Techreport is already a double of GTX 680's perceived loudness.
Imagine what happens when you overclock your Tahiti...
SLI/CF: 680 edges once again.
Nvidia has far better multi GPU support. It won't work always, but it's supported in bigger number of titles then CF.
Also, subsequent drivers do not tend to brake what's already working.
Automatic profiles downloading, and Frame Metering simply do not exist on AMD.
So even in cases when Crossfire scales better, you will be left with choppier game-play (BF3 @ [H])
FEATURES: No contest. 680 hands down!
A quick rundown on the Nvidia's exclusive bag of goodies which is getting heavier by the day:
PhysX, CUDA, GPU Boost aka overclocking for the dummies, hw acceleration in large number of applications, active sensor based TDP/OVP protection (as opposed to AMD empiric TDP limiting)
hardware-based H.264 video encoder, Ambient Occlusion, Nvidia Inspector, temperature and FPS limitter, far more robust anti-aliasing, compatibility bits, superior Full-screen antialiasing support, downsampling, improved driver FXAA, TXAA (soon to rule like the boss!), infinitely better 3D support, automatic profiles downlading, nearly shimmer-free industry leading anisotropic filtering(yeah that includes better ingame image quality - 7970 masks the AF problem via image blurring).... a working Forum, Driver Feedback and Support
Feature list is a main reason why I can't even consider buying AMD at this point in time.
I will also mention far better OpenGL support, traditionally better driver team, faster problem tackling, and a much closer relations with number of developers.
Having 3 distinct architectures in a very short period of time (VLIW4, VLIW5 and GCN), and having an imperative to support all three,
means some tough times are ahead for both AMD driver team and their user base.
In my opinion 7970 should be bought only if you can't find GTX 680, you can't live without OpenCL, or just want to make a statement.