Wait for the next gen cards to come out, they might better meet your needs (no telling for sure just yet thoguh) if you are trying to play games on that many screens. Even in quadfire, or quad sli, you won't be able to run any games at a high enough setting to justify that many monitors.
With 5760x1080, you won't even need a 6GB card setup. From what I have read, the best card out there right now is the Asus matrix 7970...but keep in mind that it is a 3 slot card, so running crossfire might cause space issues. It's too hard to tell when the next gen cards will come out, but most speculate sometime in the first quarter of 2013.