What is the difference between SATA II vs SATA III?

Transfer speed of Sata 3 - 6Gb/s, Transfer speed of Sata 2 - 3Gb/s looks like sata 3 almost doubles transfer speed.
Technically it should.

As noticed the sata cables that come with mother board are sata 2 only.

I plan to buy White sata 3, 6Gb/s cable from ebay for 249. What should i do is it worth to buy.

SATA II (revision 2.x) interface, formally known as SATA 3Gb/s, is a second generation SATA interface running at 3.0 Gb/s. The bandwidth throughput, which is supported by the interface, is up to 300MB/s.

SATA III (revision 3.x) interface, formally known as SATA 6Gb/s, is a third generation SATA interface running at 6.0Gb/s. The bandwidth throughput, which is supported by the interface, is up to 600MB/s. This interface is backwards compatible with SATA 3 Gb/s interface.

I hope you got your point.