It will make no difference at all since the maximum data transfer speed is determined by the SATA controller specification.
You can still install a 6 GB/s SSD though. It will work of course since SATA is backwards compatible, but you won't get 6 GB/s
Nevertheless, if you use the SSD as a boot drive for Windows you will still notice a vast improvement in Windows startup time because of the SSD technology itself compared to mechanical hard drives.
And any applications you install on SSD will startup faster too.