if you do the math 1.5Gb divide that by 8 gives you bytes. it's actuall 0.1875GB/s so if you do the math thats 187.5MB/s theorhetical bandwidth.
8b/10b ECC encoding reduces availible bandwidth to 150MB/sec, or 146MiB/sec, despite the 1.5Gb/sec signaling rate.
But to the OP, the *only* way SATA 3Gb, or SATA II, or whatever you want to call it, matters if if you are:
1: Using an external device over SATA that is a RAID array internally. These are rare, and the ones that I have seen are unlikely to top 150MB/sec anyway.
2: Using an SATA port multiplier to attach 3 or more disks to an SATA port. These work like hubs for USB, and are also very rare. If you have one, you'd know about it.
3: Using a pointlessly overpriced static device with hardly any storage capacity that would be better connected directly by PCI-E x4 or more anyway, like Gigabytes I-Ram device.
Add to this that even alot of *onboard* SATA ports hang off a PCI, or sometimes PCI-E x1 interface, (at 133MB/s and 250MB/s respectively) and you realise that even in those rare circumstances its kinda redundant.
I dont think a single SATA drive, even a Raptor, is going to get even 100MB/s sustained transfer. I actually think its quite a bit less than that, but I'm sure someone else has exact figures. As such, the SATA interface is NOT the slowest part of the system, so changing to SATA2 is like expanding a 100ft section of a 3 lane highway to 20 lanes and wondering why traffic doesnt speed up.