---CUT AND PASTED STAT II RANT---
To make things completely clear the existance of SATA I and SATA II drives is a complete and utter myth.
All SATA drive are of the same "SATA generation" and comply with the same SATA II specification. There are many optional features in this specification including the choice between a 1.5 Gbps (AKA SATA 150 Mbps) and 3.0 Gbps (AKA SATA 300 Mbps) interface.
Interfance choice in no way implies the presense of other optional features, nor does it have ANY EFFECT ON PERFORMANCE.
For this reason the SATA Standards board flat out forbids the use of the SATA II product label!
There is an entire thread already devoted to dispelling this myth.
CONCLUSION:
Someday in the far future, when hard drive are over 66% faster we will need 3.0 Gbps transfer rates.
UNTIL THAT FAR AWAY DATE:
When evaluating a hard drive COMPLETELY DISREGARD the interface speed or any incorrect SATA I/II labeling.
Do not waste money on a 3.0 capable addon-card if you motherboard already has enoug 1.5 ports.
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Doesn't matter at all since there is nothing suboptimal about the current 10 RPM drives, see above.