For 6 monitors you would need Eyefinity 6 OR 2 normal 5000 series cards OR 3 of any cards in non sli/crossfire would be the only way for 6.
A friend of mine uses a gtx275 sli with 2 monitors + has a 9400gt i think it is going to his tv, And in theory if you have enough pcie slots you could have 2/3/(4?) way sli/crossfire with 2/3/6 monitors and an extra card for extra monitors.
So mabey:
5870 eyefinity 6 + 3 normal/eyefinity 6 5870's + up to 3 eyefinity 6 cards/any 5000 series/any normal 2 port card for the extra 2/3/6 monitors per card, they would all have to be watercooled tho to bring them down to single slot and in a mobo like the asus p6t7, would put a hell of alot of strain on the cooling aswell, but you could IN THEORY have 4way crossfire + 24 monitors