The Apple-firmware 5770 works in all Mac Pro models.
It supports three displays if you can use ACTIVE POWERED adapters from the Mini DisplayPorts. It runs slightly slower than expected on the Mac Pro 1,1 and 2,1 due to PCIe 1 slots instead of the later models' PCIe 2 slots. It will support up to three 30" displays.
US$250 from the Apple Store,
or a little less (US$244.80) from B & H in New York city