Best Graphics Card? Upgrading Mac Pro.

Hi,

I don't know too much about video/graphics cards so need your help.

If I am looking to spend about $600 on a video/graphics card, which would you recommend? I think NVIDIA's GeForce GTX 680 is better than any of the competeing AMD cards or other cards, but am unsure. I would like to use the card for hardcore gaming and multitasking, possibly with dual monitors. Although one graphics card may not be sufficient, I just want to know which is the best card for $600.

Thanks!!