SATA 2 (3Gb/s) speeds are from 151MB/s to 300MB/s.
SATA 3 (6Gb/s) speeds are from 301MB/s to 600MB/s.
The WD Caviar Black has data transfer speeds of 126MB/s.
The Seagate Barracuda has data transfer speeds of 72MB/s.
Yes, there should be some real-world difference if you upgraded to the Caviar Black.