I originally started with a single 680 at 1080p 3D gaming, and found 3D was worth far more than max settings. For starters, it is easy to turn off a few things in 3D, that don't add much and maybe even a detriment to 3D. DoF is not needed in 3D as it tries to create a 3D affect, so there is no point when you already get depth. AA is half as useful, as you get 2 images that mesh together, it is almost like having twice the resolution, the same can be said for textures, though I still like to use those at high when I can.
I'd honestly would take medium settings in 3D over Ultra in 2D, and I did for a while in some games like Metro 2033. I also always play at 60 FPS, so if you don't mind lower FPS, you might be able to up that some.