EyeFinity (3 monitors) CrossFiring two video cards will make them act as a single video card so at most you can only connect 3 monitors. If you install another HD 6950 and do not setup CrossFire, then you can have up to 6 displays.
This is actually wrong. The 5000 series this was true but the 6000 series you can connect up to 5 if you have the right card. I have 2 6950's and the primary is running 4 monitors so I can maintain X-fire on with 4 monitors. I will go grab another monitor to verify the fifth but there are 5 active outputs on my ASUS Direct CUII cards.