You can buy and install an internal SATA II controller card in your PCI slots. For example, I searched on the egg for SATA II controllers for the PCI Express slot (PCIe x1 is better than PCI because the communication speed is higher and matches the speed of SATA drives, and many systems have such slots with no card in them.) Here's what I got - 3 pages of them, priced from $20 to $400.
http://www.newegg.com/Product/ProductList.aspx?Submit=ENE&N=100007607%20600022627%20600022730&IsNodeId=1&bop=And&ShowDeactivatedMark=False&Order=PRICE&PageSize=20
You do NOT need fancy, and you probably do NOT want fancy RAID features (simple RAID seems to be common, but you don't have to use it), so concentrate on a RELIABLE card. You may need to buy one or two SATA data ribbon cables. Also, you will need at least one SATA power supply connector. If your PSU does not have these already, get an adapter that converts a common 4-pin Molex power connector to SATA.
In choosing a SATA card to add on, there's a little wrinkle to check. Your existing system's BIOS has no SATA features, so it will not know how to boot from a SATA drive. However, some mobo BIOS's DO allow for having a controller card added to the PCI slots that carries its own extension of the BIOS, and it is able to have that extension just add on to the total system ability. So, IF your mobo BIOS can let add-on cards do that, then check to be sure the card you plan to get does have its own extension of BIOS to allow the system to actually boot from a SATA drive connected to the card. All of this, of course, assumes that you might want to actually do that at some time, and not be limited to using the SATA drive only for data storage.
You do NOT need fancy, and you probably do NOT want fancy RAID features (simple RAID seems to be common, but you don't have to use it), so concentrate on a RELIABLE card. You may need to buy one or two SATA data ribbon cables. Also, you will need at least one SATA power supply connector. If your PSU does not have these already, get an adapter that converts a common 4-pin Molex power connector to SATA.