Buy 2x2TB drives, Replace your current two drives.
Not at current prices. Have you seen the price of 2TB drives this week?
But yeah, a cheap SATA controller card might cost you $25. A good one, a little bit more.
If you're still stuck in the stone ages of using PCI though, I feel for you man. Do modern boards even have PCI slots anymore? Saw that new MSI X79 board has seven x16 PCI-e slots! (Though I think maybe only 2 are x16, so it's 2 x16s and 5 x8s).