For high speed SATA, you'll probably want to use GTP transceivers, and if you have those, you might as well use PCIe. I'd probably recommend getting something like an SP605 (supports x1 Gen1 - 250 MB/s) or ML605 (x8 Gen 1, but a lot more expensive!). You can use partial reconfiguration to avoid rebooting the PC between bitstream changes, though this isn't well supported on the Spartan-6.
What are you planning to do with the 3 Gbps of data? I did something similar using UDP over gigabit Ethernet at about 700 Mbps and my PC struggled to keep up with that.
USB 3 might be another option if you can find a PHY that works well with the Spartan-6 SelectIO.