Be aware, however, that almost all SATA drives (including SATA II) can't actually deliver the 3 Gb/s data transfer rate. That speed is the MAX transfer speed for data from the drive unit (really, from its buffers) to the PC mobo. The real limit, however, is how fast the data can be read from (or written to) the spinning disk inside, and that tend to average out less than 1.5 Gb/s, the max rating of the first version of SATA. That's not a lot different from the speed rating of the PCI bus, so it may not be much of a limit.
This argument does NOT apply to SSD units, of course, in which there is no spinning disk to limit the data rate. Those units are the ones that will really benefit from the new SATA 6 Gb/s standard.