Question : Best use of a SATA 6g/s hard drive?

I'm setting up a system with three hard drives:
Western Digital Caviar Black 1.0TB SATA 6 Gb/s
WD Caviar Black 1.0TB SATA 3 Gb/s
WD Caviar Green 1.5TB SATA 3 6Gb/s.  

The mobo has SATA ports for both speeds.  Are there any circumstances where the higher potential transfer speed of the 6Gb/s drive would actually come into play?  

I'm trying to decide whether this should be the primary (system) drive or a storage drive, and not sure if it would make a difference.  Thanks.

Answer : Best use of a SATA 6g/s hard drive?

Two places it would come into play:

1.  OS Drive - If you will be doing mostly applications and Internet browsing, make it your OS drive
2.  Swap File - If you plan on doing a lot of memory intensive work that will generate swap activity, put the swap file on a partition at the beginning of the drive and use the rest for data.