Well comparing Western Digital drive at the 200gb size:
Western Digital Caviar SE 200GB 3.5" SATA 3.0Gb/s Hard Drive ($91)
Western Digital Caviar SE 200GB 3.5" Serial ATA150 Hard Drive ($92)
You are right about not seeing a huge increase in preformace, there will be an increase though. But at this price, the 3gb/s SATA is a better buy. It is true that a faster transfer rate wont make the drive any faster, but the drives already opperate at above the 1.5gb/s transfer rate.
Which is the same reason that cd and dvd burners havent gotten any faster, ATA100 can only transfer so fast, once they go to SATAI or SATAII for optical dirves you will see a increase.
Even if there was a price difference, upwards to 20$ I would still consider buying the SATAII drives. Future-proofing my pc for when I finaly build a new pc with SATAII, thus, its one less piece of hardware to rebuy, or have to use slower hardware.