Does it matter at all if my mobo & processor are SATA III 6 Gbps or SATA II 3 Gbps if all Im going to use the regular Seagate/WD 1 to 2 TB capacity HDD's (that use mechanical parts and 7200 rpm etc)? If my HDD is going to be a bottleneck I dont want to spend extra on a SATA III capable mobo and processor.
If yes what should be the specifications of the HDD's (the regular 7200 rpms ones that use mechanical parts) to obtain SATA III 6 Gbps speed?
Even the HDD's listed as Sata 6gb/s don't actually use the connection to anywhere near it's potential. Getting a mobo for Sata III and not having an SSD or HDD capable of using it is a waste of your money. Most lower end SSD don't even need Sata III and they are many times faster than a HDD.
SATA-III was developed because the very fastest SSDs were bottlenecked by SATA-II. But hard drives just don't spin fast enough to get anywhere near even SATA-II speeds, so they don't benefit from SATA-III.
The manufacturers who build the SATA interface chips are all switching over to SATA-III, so most of the new drives coming out use those chips whether they need it or not. It's the same situation as with USB - all mice and keyboards now use USB 2.0 even though USB 1.1 was more than fast enough for them. But nobody makes USB 1.1 chipsets any more, so even pokey slow devices are using the 60MByte/sec USB protocol. In fact they'll probably all end up using USB 3.0 at some point in the not-too-distant future.