Is that such a thing even exist ? hoping for a 60fps (so it don't drop below 30fps during stress time). I'm asking for 2x set-up so I could just buy a third one in 2 or 3 years to keep my PC able to run 3x monitor at max graphix on any game.
Just to clarify what already been said. Crossfire support a maximum of 4 cards and dual cards like the 6990 count as 2. You can crossfire a 6990 with 1 or 2 6970s. All cards only take 1 slot but all high end cards take 2 slot spaces so some boards that allow 3 cards cannot fit 3 because of the size. As stated crossfire does not scale that well beyond 2 cards but as with crossfire in general it varies from game to game.
I recommend get a 7970 and if you want more power get another later, or wait a month or to and see what the 7950, 7990 & Nvidia cards are like.