Remember those 30" displays need dual dvi links to drive them.

An Nvidia Quadro4600 will drive 2 of them, or with cable splitters drive 4 of your current displays. Uses one pci express x16 slot, but is double width.

~$1600 US. Amazon

I use one of these with a 3007WFP for cad. Same # pixels as 2 20" 2007fp monitors.