Get a Radeon 7970. Faster than GTX 680 & 670, Overclocks like beast!
But check out performance of 7970 in 3D programs because Nvidia has cuda which is supported by most 3d apps.
I've heard alot about GTX 580, its good for rendering and all other stuff in 3d apps.
So main competition is between GTX 680, GTX 580 and 7970.
GTX 670 is a good card, may OC better or something but when it comes to Programming then its a little better to get more Cuda cores.
So go ahead and check other Benchmarks on Maya & Mudbox.