Well it seems you have four options.
7970 GHz- http://www.newegg.com/Product/Product.aspx?Item=N82E16814202001
Now for the explanation. First the 670 is the relative worst of the group. The 7970 and 7970 GHz are both the same GPU (a bit ahead of a 670) but the GHz has higher factory overclock settings, enough that they released it as a second model. The regular 7970 can run at those same settings though (normally at least, some don't overclock as well but almost all do) if you're willing to overclock saving you some money. Then there is the 680 which performs similar to the 7970 GHz or the overclocked 7970.
Finally you have to decide which brand you prefer for both 3D and modeling software. The 7970 scores higher in cad benchmarks but the Nvidia cards support CUDA.
I can't personally comment on their of these though.
Here is an article about the 3D (though a bit old now, october 17,2011).http://www.tomshardware.com/reviews/geforce-3d-vision-hd3d-steroscopic,3050.html