I have just upgraded to a WD Caviar Black drive capable of 6.0Gb/sec transfer.
'Capable of' doesn't meant it is 'able to'.
Mechanical hard disks are limited in their transfer speeds by their own physical actions. SATA 6Gb/s is pretty pointless for mechanical hard drives as they are simply not fast enough to utilise the bandwidth available. So no, there would not be any performance increase even if you added a SATA 6Gb/s SATA controller card.