If you want that kind of capacity, I'd build a dedicated NAS computer. Here is my choice for a SATA RAID card:
http://www.newegg.com/Product/Produc...82E16816103010
This card performs best in a PCI-X slot, so I'd build around this motherboard:
http://www.newegg.com/Product/Produc...82E16813182105
I'd get 8 drives, set 7 up in RAID 5 with the 8th for a hotspare. The array capacity with RAID 5 is total number of active drives minus 1. This would give you 4.5tb with 750gb drives.