Pure guess, adding the other card is effecting the RAID due to limitations of the board. Its an older board, and probably can't maintain that many connections. You can try disabling things in the bios that you don't use like firewire, parallel ports, etc. You can also try a different PCI slot if possible in case its an IRQ problem.
It not, dump those Nvidia cards and get some newer AMD ones. The Eyefinity cards from the 5 series support 3 monitors each, while newer 6 series cards can do 4 each.