I built a computer last year (first build since I was in the ninth grade back in the 90s). It was for my partner, so nothing fancy. Core i5-661, ASUS P7H55-M-Pro, 4BG ddr3, x2 1TB HDDs.
Now, she has gone to a laptop and so I have the computer. At some point, I want to build a 'good' computer for me (whatever that is) and to build a NAS for backup solution (14TB worth of HDDs to be put on my computer). I'll probably end up using her computer as the guts for the backup.
In the meantime however, I want to put some 6gbps 2TB HDDs in my partners computer. Will the MB in it support 6gbps?
Unless you are looking at SSD drives it doesn't make any sense to use SATA3 ports on them, and if you're looking as I assume your are at Western Digital and want RAID then you're stuck to their Enterprise line for anything requiring parity. A SATA3 HDD/SSD is backwards compatible to a SATA2 port, and a SATA3 port is backwards compatible to a SATA2 HDD/SSD
1) 3gb and 6gb is forward and backwards compatible; no problems there.
2) The 3gb or 6gb applies to the transfer rate between the hard drive buffer and the sata port. That indeed may proceed at 3gb or 6gb. But, a hard drive will transfer data from the platter at a much slower rate, depending on the density and rotation rate. That will be something on the order of 100-150mb. UNless you are simultaneously transferring data from many drives, there will be no performance impact.