you'll need a new motherboard that has the SATA III controller chip, whenever that gets released. but really im not sure hard drives are even at the edge of SATA II speeds so i dont know how much of a difference it makes.
SATA II nor III does not exist. That naming has been abandoned by SATA-IO long ago. Calling it SATA II or III can just mean SATA.
The correct way when speaking about SATA rev. 3.0 is "SATA 6Gb/s" (note the small case 'b' which indicates bit and not byte).
Anyone who tells you SATA II or III is speaking bollocks.
As all above posters have said, SATA 6Gb/s requires a new SATA controller, either as integrated into chipset/southbrige of motherboard or as expansion card.
There are already SSDs made with PCI-e interface for 2GB/s+ full duplex bandwidth, actual speeds 1GB/s+. But these are expensive enterprise class products, not available for mortals like you and i. But this kind of technology could be commonplace and would be logical for the system disk. See here for an example:
But chipsets with 6Gbps SATA and also USB3 (10 times faster than USB2; 4,8Gbps) are coming with the next generation AMD 800-series chipset, and nVidia/Intel will likely follow. Another feature is Powered eSATA, removing the need for seperate power plug for eSATA devices. One cable supplies both data and power.