Uh, what?
Are you serious?
NVIDIA's R&D is far more advanced. Their architecture is head of AMD's by a long shot.
NVIDIA has both power & complexity on their side. Their architecture is geared towards the future, wile AMD is trying to stay afloat.
AMD's stock dropped 2% when the 680 was released, and since then has gone to 4%.
If you read about the internal designs, it's amazing what NVIDIA is pushing.
I'd say NVIDIA is better, but they cost more, but their R&D is like 11 Billion while AMD's is...2 Billion, I think, so of course Intel has better toys. AMD also can't get drivers right. NVIDIA's not perfect, but you can usually play a game with a NVIDIA card upon release, while my friend with a 5970 still has trouble.
And the 7990 will be x2 7950's, so yeah, I'd damn well HOPE it beats a single card...be massively disappointing if it didn't. To everyone, even if you don't like AMD.
The 680 is also considered mid-end for NVIDIA, with the real Kepler launching later this year.
The 690, which will be 680'x2, will smash the 7990.
NVIDIA has better pricing, better performance, less heat, and less power usage...and more features, such as CUDA, PhysX and far more advanced OpenCL support.