2x video cards dont mean 2x more fps or performance...
Example( that depends strictly from the games that u play.Dont expect to have 170 fps on Far Cry 3/4
)
1 GPU- 100 fps
2 GPUs in crossfire 150-160 fps max (1 Gpu + ~50% from the second one..)
Video card memory dont stack..if cards are with 3 gb each.Each GPU is going to draw 3 gb vram max to laod textures
I dont know how it works on Eyefinity