ok. since no one answered, I will explain after my own investigation.

SLI 1.5GB x2 != 3GB

The limitation of SLI is that each GPU can only work with its own VRAM. So Card A cannot work with card B 's memory. and When both cards are working, pixels are duplicated in both cards memory for each GPU.

So I will never get 3GB worth of pixels in my GTX 590. It is a 1.5GB card, not 3GB. True SLI works the same. if you do sli 570 x2, you still only have 1.25 GB VRAM.