I use 1 card per monitor. Card1/monitor1 is dedicated to my master, Card2/monitor2 is for my slaves, browser and chat.

I couldn't speak to gain. My husband bought me 2 cards so I could run SLI for games I don't multibox.

However, I have noticed a few things. I can't run both master and slaves in DX11 for WoW. Trying to put the slaves into DX11 keeps sticking my slaves in 800*600 and running it from the primary card/screen. And because it's trying to run on the wrong card, when my screens get bumped over to the proper screen, I see a huge performance hit.

But it works just dandy with the master in DX11 and slaves on DX9 or everyone on DX9.