Dereck47's reply hits the three important points; +1.
I am running my machine with an SSD on an SATA II port. Its performance is amazing compared to when it ran from a Velociraptor.
While there are many SATA III add-in cards sold, they tend to be PCI-E x1, which is silly because it's slower than a properly connected SATA II port. I haven't seen many PCI-E x4 cards for SATA III.